Increase kFastTrackMultiplier from 1 to 2

Change-Id: I158f147295eebcea96e4047d7618069bc48bdd7d
stagefright: Don't enforce 16kHz rate for AMR-WB

This breaks android.media.cts.MediaRecorderTest#testRecordingAudioInRawFormats,
which tests specifically with 8kHz.

Change-Id: Ibfa54db4b2a82e502268be18f4ea405a41a88bfd