Architecture summary:

1/ frameworks/base/core/java/android/speech/tts/TextToSpeech.java
This is the public API for TTS. It uses a TTS Android service whose interface 
is described in: frameworks/base/core/java/android/speech/tts/ITts.aidl
All TTS calls an app makes through the TextToSpeech class are routed to the 
service (see the mITts member variable)

2/ frameworks/base/packages/TtsService/src/android/tts/TtsService.java
This is the implementation of the TTS service. It handles the speech queue, 
and is responsible for instanciating the actuall speech synthesizer, an 
instance of the SynthProxy class, implemented in: 
frameworks/base/packages/TtsService/src/android/tts/SynthProxy.java

3/ frameworks/base/packages/TtsService/src/android/tts/SynthProxy.java
This class, when instanciated, is given the full path of the speech synth 
native library to load. It is the bridge between Java and native code.

4/ frameworks/base/packages/TtsService/jni/android_tts_SynthProxy.cpp
JNI for the SynthProxy class. It loads the synth native library whose 
interface is defined in frameworks/base/include/tts/TtsEngine.h.

5/ external/svox/pico/tts/com_svox_picottsengine.cpp
An implementation of TtsEngine.h which is built into a shared library, 
loaded by android_tts_SynthProxy.cpp

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Here are the steps needed to create a TTS plugin that will work with the 
Android framework:

I. Build the .so file

  The main piece of work that you need to do is write an equivalent of 
  "com_svox_picottsengine.cpp", ie "com_mytts_ttsengine.cpp". This file 
  must implement "TtsEngine.h" as seen here:
http://android.git.kernel.org/?p=platform/frameworks/base.git;a=blob_plain;f=include/tts/TtsEngine.h;hb=master 

  Use the Android NDK to build your .so file.
  See http://developer.android.com/sdk/ndk/index.html

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II. Package it inside an Android apk file.

The Java APK wrapper MUST implement the following Activities:
<NameOfEngine> - Declares which languages are potentially supported, name of 
the engine, etc. This is a dummy Activity that is used to get some basic info 
about the engine.

DownloadVoiceData - Downloads data; this is the Activity that will be called 
if the engine is selected under TTS Settings and it needs voice data.

CheckVoiceData - Determines which voices are actually present on the system. 
This will be used to determine which Languages the user can choose from in 
TTS Settings.

GetSampleText - Sample text to be spoken to the user when they click 
"Listen to an example" in TTS Settings.

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The Java APK wrapper can implement this Activity if it has Settings 
for the user to configure:
EngineSettings - Engine specific configuration. This is the Activity 
that will be called if the user clicks on the engine specific settings 
under "Engines" in TTS Settings. What the user sets here will be passed 
to the .so as one String; it will be retrieved by the TTS Service by 
querying it from the engine's providers.SettingsProvider and then passed 
back to the .so in the native layer. If this is not implemented by your engine, 
the area under "Engines" will only have the enable/disable checkbox for 
your engine; there will not be a settings entry for your engine.

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The Java APK wrapper can also implement this Content Provider:
providers.SettingsProvider - Provides the String of config data that is to 
be passed to the engine's .so file. Normally this would be some data that 
you get from what the user has set in your EngineSettings Activity. The 
framework does not attempt to parse this data or do anything with it. 
It is up to the plugin vendor's Java wrapper to come up with this String, 
and it is up to the plugin vendor's engine .so to consume it. If you don't 
have this Content Provider, the native layer will just get an empty 
string passed to it.

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For apps that use TTS, we expect that developers will probably want to use 
the dummy NameOfEngine Activity to narrow down which engines have support 
for the languages they are interested in, followed by invoking CheckVoiceData 
for the specific languages they are interested in.

For examples, see: 
http://code.google.com/p/eyes-free/source/browse/#svn/trunk/documentation/TextToSpeech_Plugin_Engine_Examples
    
Also, for a 3rd party example, see Flite for Android: 
http://github.com/happyalu/Flite-TTS-Engine-for-Android
    
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III. Stress test your TTS

Make sure your TTS can handle starting and stopping speech rapidly. The easiest
way to stress test your TTS is to use it with Android's built in TalkBack 
screen reader application and scroll around the various menus in Android. To 
enable TalkBack: Settings > Accessibility > TalkBack.