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

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

Change-Id: Ibfa54db4b2a82e502268be18f4ea405a41a88bfd
Merge pull request #2 from CyanogenMod/cm-11.0

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