stagefright: add SimpleDecodingSource

SimpleDecodingSource is a lightweight replacement for OMXCodec
for decoding use. It does not support pause, secure content or
precise seeking. It also does not support start after stop.

Bug: 17108024
Change-Id: Ic310f9372ebf214cbd73118b18e5d2abcc4426cf
(cherry picked from commit 25a21c4b)