libstagefright: Fix crash when generating thumbnails for AVI & h264 clips

If parser fails to identify a sample to use for thumbnail generation and
the client doesn't want thumbnail from any specific timestamp (i.e.
passes in negative timestamp), an assertion in software decoder will
fail because the software decoder attempts to start decoding from a
negative timestamp.

So instead, we'll pick up the thumbnail from timestamp 0, which we
expect to be an i-frame.

CRs-Fixed: 284873
(cherry picked from commit 23d7b006f81ac2ffbc3866f0a730a6ce823763c6)

Change-Id: I626fc8e322ceb2b2d9e848ebc12423e6f739f635