av: Clean up QCOM build flags

 * Remove dead flags
 * Enable standard flags by default

Change-Id: I15b4743a2569360dcb5798f2f1d8f4390a68899a
libstagefright: ifdef the audio_defs.h include for QCOM_DIRECTTRACK

Devices that use BOARD_USES_LEGACY_ALSA_AUDIO do not use this file,
this was breaking build for all 8960 devices.

Change-Id: Ic0ae606615f7ccc72cef9836d52bfa7045f326ae