stagefright: change signature of MediaCodec::Create methods

Most clients already have AString when creating codecs, and
internally we use AString.

const char * automatically casts to AString, but it does incur a
copy cost, so we incur it on every MediaCodec::Create call.

Bug: 17108024
Change-Id: I213dde7a6fc3cf3756eee6afd7194413bcca54a5