Add new sound effect constants and make touch_sounds group optional in audio_assets.xml

Add sound effect constants for back, home and fast-scroll.
The touch_sounds group name is now optional, which makes the
audio_assets.xml file more generic.

Bug: 157407957
Test: manually tested on a device
Change-Id: I97ee1920032b8db46acea29ed5693a4b63d7dfcf