Use audio_source_t consistently

Was a mix of audio_source_t, uint8_t, and int.

Related fixes:
 - fix comments in MediaRecorder.java
 - AudioPolicyService server side was not checking source parameter at
   all, so if the client wrapper was bypassed, invalid values could be
   passed into audio HAL
 - JNI android_media_AudioRecord_setup was checking source for positive
   values, but not negative values. This test is redundant, since already
   checked at Java and now checked by AudioPolicyService also, but might
   as well make it correct.

Change-Id: Ie5e25d646dcd59a86d7985aa46cfcb4a1ba64a4a