SimpleDecodingSource: fix handling of Vorbis buffers

The Vorbis decoder needs the number of samples in a page added to
the end of the encoded data, which SimpleDecodingSource wasn't doing.
This affected Vorbis decoding using OpenSL ES.

Bug: 32258336
Test: ran CTS and OpenSLES tests
Change-Id: I436e4a1ee998ce4581eb540bea8b66154960a4e7