IMS-VT: Handle power button interactions in a VT call

1. Introduce InCallUiStateNotifier class that listens to below events
   and notify whether UI is visible to the user or not using
   onUiShowing API
       a. InCallActivity's lifecycle events (onStop/onStart)
       b. Display state change events (DISPLAY_ON/DISPLAY_OFF)
2. Introduce InCallUiStateNotifierListener Interface that exposes
   onUiShowing API
3. Handle onUiShowing API to
       a. Turn ON/OFF camera
       b. Send Pause/Resume requests when multitasking is enabled
4. Ignore duplicate Pause/Resume requests

IMS_VT: Enable camera only when InCall UI is in foreground

- We don't need to enable camera when InCall UI is in
  background.
Change-Id: Ic95b4ab8d2e8e3a8ba0ea856a53bb320aa713e37

IMS-VT: Always update "mIsInBackground" flag

update "mIsInBackground" flag whenever there are indications
that UE moved to either background or foreground and never
ignore these indications
Change-Id: I9f127fa640b779175dda95ff44aa5e77dc645a42

Change-Id: If4da4fa9fa3a007d23d1bf9e9896e6f40d859900
CRs-Fixed: 881797