Bug 4016329 do full string comparisons

Use full string comparisons instead of partial for
  file extension and MIME type.
Do case-insensitive comparison of MIME type and file extensions.
Fix error in comment for String8::getPathExtension.
Remove dead code -- StringTokenizer is unused.

Change-Id: I322be6235abbdaab5f7eafa48926dbb2cf46dc29
6 files changed