[3/3] InCallUI: Flip to Mute/Reject Call

Actual Logic for this functionality.

PS2: Remove unwanted space

Base: https://gerrit.omnirom.org/#/c/4668/
Telephony: https://gerrit.omnirom.org/#/c/4670/

Change-Id: Ia10df0045e9919c6b7efc97857abaeba246c365d