Bug 890434 - Impossible to silence an incoming call without rejecting it

We use Inter App Communication to make this work
- System app sends a message in 'dialercomms' when we press the power or volumedown buttons
- The Call Screen stops ringing and vibrating when receiving this message

I moved the Audio mock to shared because it is now used by 3 apps.
Had to modify some sms tests to work with the new shared mock.
Merge pull request #14577 from Rik/stop-ringtone-power-button-890434

Bug 890434 - Impossible to silence an incoming call without rejecting it r=etienne