avcenc: Only do startcode escaping if the next byte requires it

Section 7.4.1 in the H.264 standard says that the only valid bytes
to follow a sequence that starts with 0x000003 are 0x00, 0x01,
0x02 or 0x03.

This makes EncodeDecodeTest pass properly when decoding using
OMX.google.h264.decoder, which is strict about the forbidden
escape sequences.

Change-Id: Ice113d9b934015003ea9cb10d0b21cee4d18d774