libstagefright: Fix incorrect comparison of mimetype

'sizeof' is used to calculate length of string constant which returns
size of pointer to string constant. This causes comparison of only
four characters of mimetype. By using strlen, entire mimetype string
is compared - which returns correct result.

Change-Id: I4d081d3452f167d49aebbe3219e8e522ec29d5be
CRs-Fixed: 664311