AudioAttributes: add ability to set Hotword mode for capture

Allow setting Hotword mode for capture usecases that want to use the low
power DSP based hotword source for capturing audio. This will be only
available for AudioSource.VOICE_RECOGNITION. There is no guarantee that
this mode will be available on the device. The caller must have the
CAPTURE_AUDIO_HOTWORD permission granted.

Bug 178317696
Test: Able to setHotwordMode from AGSA application. Confirmed
HAL receives the flag and audioSource correctly in open_input_stream.

Change-Id: I1fb80a047654e2abb8a587bf2f6ff82fcacd67b6