Make StatusBarWindowController aware of assist session in progress.

Separate "assist is rendering something" from "assist session is in
progress". If an assist session is in progress, the status bar should be
focusable and back gestures should cause the session to be canceled.

This also intercepts touches outside of the status bar when a session is
in progress, but those touches will cancel (separate CL), so this
shouldn't be a problem.

Bug: 123413054
Test: Ensure in-progress assist sesion is canceled on back, ensure back
is not affected outside of a session.

Change-Id: Ife24b58a7851c5865ff75e66e00e32ae3293a0e8