Forward back key event to host app for SCVH.

Forward the back event to host app for SCVH by default. So host app
can decide what to do, also app developer can register their own
callback if they want.

And since the VRI of host window will not have mHasWindowFocus, set
mProcessingBackKey flag to make sure the VRI won't drop back key event
while dispatch back key event to it. This would be needed either back
event is triggered from gesture or back key.

Bug: 305924324
Test: force enable OnBackInvokedCallbackEnabled, verify the host
activity can be closed when back event is dispatch to remote view.
Test: disable OnBackInvokedCallbackEnabled, verify above test pass.
Test: also verify above cases can pass when either trigger from gesture
or back key.

Change-Id: I50bf5a259b2c80aab896d4c7fc38e28e0b22b252