system and UI sounds volume policy

Implement a more consistent policy for system and UI sounds (key clicks, lock/unlock,
camera shutter, DTMF, low battery...):
- All system sounds are played over STREAM_SYSTEM stream type.
- The STREAM_SYSTEM volume that was previously fixed now tracks the volume of a "master"
stream type. This "master" stream type is STREAM_RING for phones and STREAM_MUSIC for
tablets which corresponds to the stream whose volume is modified by default by the volume
keys.
- The STREAM_SYSTEM volume ranges from -24dB to -6dB (-24dB to -12dB over headphones) when the
"master" stream volume ranges from its min to its max.
- DTMF tones are played over STREAM_DTMF that tracks the "master" stream volume in the same
manner with the following exception: when in call, DTMF stream tracks STREAM_VOICE_CALL volume.
- Camera shutter sound is played over STREAM_SYSTEM_ENFORCED stream that tracks the "master"
stream volume except in countries where regulation enforces this sound. In this case
its volume is fixed and cannot be muted.
- Low battery sound is played over STREAM_SYSTEM and therefore has a tunable volume and is
heard while in call.

Issue 6344620.
Issue 6069229.
Issue 6213100.

Change-Id: I53a237878ead596e706c5dbbb1420e62cde32bd7