stagefright: send EOS to encoder for thumbnail extraction

Decoders might hold on to decoded frames for certain streams,
send EOS after last sample so that we get all output frames.

bug: 110116757
bug: 110226430

Test: CTS HeifWriterTest; MediaMetadataRetrieverTest
Change-Id: I0f8e7f1607857297325594adad8ca4fbff16e5e9