audio_policy: Allow a sample rate of 8 KHz only for VoIP output

Audio library allows only a sample rate of 8 KHz for VoIP output, when
a different sample rate is used, for example 16 KHz, the following
errors occur:
D/AudioHardware7x30(  156): AudioHardware::openOutputStream devices 1
    format 1 channels 1 samplerate 16000
...
E/AudioHardware7x30(156): AudioStreamOutDirect::set return bad values
E/AudioHardware7x30(156):
E/AudioHardware7x30(156): set Failed for AudioStreamOutDirect

After encountering these errors, some apps downgrade to a sample rate of
8 KHz while others stop working.

By removing the sample rate constraint on audio library, it's possible
to use a sample rate of 16 KHz with some apps like Hangouts and Viber,
but other apps like Skype and Line won't work properly anymore.
So it's better to keep the audio library as is and to use a sample rate
of 8 KHz only.