modifications for new system volume policy.

The new system and UX sound volume policy makes that STREAM_SYSTEM
volume is not fixed anymore. It is tracking STREAM_RING (phones) or
STREAM_MUSIC (tablets) in a constrained range between -24dB and -6dB.

Sound Fx where previously played over STREAM_SYSTEM with a fixed
attenuation of -20dB. The default value of 5% in the keypress sound
volume setting was corresponding to -26dB, meaning 6dB below default.

Modified the default volume value to 50% so that by default, keypress sound
volume varies from -30dB to -12dB which is also 6dB below the other system
sounds.

Change-Id: I146f72275b8e88fdce5ccf8b6dae2903c27f15eb