AudioPolicyManager: Use "safe" speaker for notifications if available

On particularly loud devices, unexpected audio bursts can be harmful.
For longer audio events, the conventional mitigation strategy is to
somehow soft-start (ramp) the loudspeaker volume, but this strategy
can severely impact the audibility of short audio events.

Use of the "safe" speaker path hints to AudioHAL to immediately output
at best allowable level for transient audio events.

SPEAKER_SAFE is aliased to SPEAKER for purposes of volume control.

Bug: 17319721
Change-Id: I75187c23f3d1f18e9f638c5d14e7b0bf805a67bc