Use channel mask instead of channel count for track creation

Record and playback objects (resp AudioRecord and AudioTrack)
are created using a channel mask, but this information is lost
in the mixer because only the channel count is known to
AudioFlinger. A channel count can always be derived from a
channel mask.

The change consists in:
- disambiguiting variable names for channel masks and counts
- passing the mask information from the client to AudioFlinger
 and the mixer.
- when using the DIRECT ouput, only verifying the format of
 the track is compatible with the output's for PCM.

Change-Id: I50d87bfb7d7afcabdf5f12d4ab75ef3a54132c0e
diff --git a/include/private/media/AudioTrackShared.h b/include/private/media/AudioTrackShared.h
index 1827c3e..072329d 100644
--- a/include/private/media/AudioTrackShared.h
+++ b/include/private/media/AudioTrackShared.h
@@ -82,7 +82,7 @@
                 // 16 bit because data is converted to 16 bit before being stored in buffer
 
                 uint8_t     frameSize;
-                uint8_t     channelCount;
+                uint8_t     pad1;
                 uint16_t    bufferTimeoutMs; // Maximum cumulated timeout before restarting audioflinger
 
                 uint16_t    waitTimeMs;      // Cumulated wait time
@@ -90,6 +90,7 @@
     volatile    int32_t     flags;
 
                 // Cache line boundary (32 bytes)
+
                             audio_track_cblk_t();
                 uint32_t    stepUser(uint32_t frameCount);
                 bool        stepServer(uint32_t frameCount);