Improved native TTS interface header.

The purpose of this patch is to add a C-based ABI that can be exposed by the NDK
for developers wanting to write a TTS Engine. This replaces the C++ ABI that is
currently under frameworks/base/include/tts/TtsEngine.h but is *binary* compatible
with it.

As a consequence, the svox pico plugin under external/svox/pico/tts/, which
links against tts/TtsEngine.h can be loaded by the TTS service properly.

Another patch would modify the pico tts to use <android/tts.h>, then we will
be able to get rid of <tts/TtsEngine.h> in the source tree.

Change-Id: I16844cef9b5b006cc32655a29e4f9f193c8c1a91