Bug 3376700 Add support in APM for stream-specific volume curves

The stream volume was handled the same way for all different stream,
 the only potential difference between each of them being the number
 of steps available to the user to change the volume. This was
 mapped to 99 steps of 0.5dB amplitude, offering a maximum attenuation
 of -49.5dB.
This change consists in defining for each stream a curve with two
 knees (3 segments) for conversion from volume index to attenuation.
 This curve is defined in the AudioPolicyManager in
 initializeVolumeCurves(), and can therefore be overridden by the
 platform.
Note that this change doesn't modify the volume curves: this CL
 enables the curves to be changed by overriding this default
 behavior.

Change-Id: I575b66799c52df2906db248943b15120b8a79ea2