Implement new NDK image decoding apis

Bug: 135133301
Test: Ib84462ea5fa8a7779eaa44494775e182e52ecaca

Separate out the code for encodedFormatToString into a piece which
returns a const char* that can be used by AImageDecoder (with its own
header) and the part that the JNI code uses to convert that into a Java
String.

Change-Id: I4cf8bfb0aacfb8e22c3f9b1689bd614ed1253673