audio: Fix for QCOM's HW AAC encoder

- CTS test(testAACEncoders) fails due to limitation of
  QCOM's HW AAC encoder. So, removed it from
  media_codecs_8960.xml to use Google's AAC encoder
  by default
- Also added entry in system.prop to enable QCOM HW
  AAC encoder: qcom.hw.aac.encoder=true
- QCOM's HW AAC encoder is enabled in frameworks code
  when qcom.hw.aac.encoder=true

CRs-Fixed: 509575

Change-Id: I3900aeb24a7365ac08e21e94813faec6adc82779

Conflicts:
	config/media_codecs.xml
	system.prop