defensive parsing of mp3 album art information

several points in stagefrights mp3 album art code
used strlen() to parse user-supplied strings that may be
unterminated, resulting in reading beyond the end of a buffer.

This changes the code to use strnlen() for 8-bit encodings and
strengthens the parsing of 16-bit encodings similarly. It also
reworks how we watch for the end-of-buffer to avoid all over-reads.

Bug: 32377688
Test: crafted mp3's w/ good/bad cover art. See what showed in play music
Change-Id: I802a71c6e5968aefde21eb1612b720ff6d579988