audio: fix audio recording failure

- AudioPolicyManager picks up 5.1 channel mask which
  is the last entry of input profile channel masks.
  However, 5.1 surround sound recording is supported
  only when corresponding system property is set.
- Fix this issue by setting AUDIO_CHANNEL_IN_FRONT_BACK
  as the last entry of the input profile which is supported
  by default.

Change-Id: I5212a2dcf168b135870bac3e1163feecc0f6849d