Do not change focus when back gesture starts

...because that may cause unnecessary focused app change.

We now just replacing the back-invoked-callback to the
right window while keeping the focus unchanged. With
that, the ViewRootImpl is no longer dropping back event
when the window is not focused because it is now a valid
use case.

Bug: 351071991
Test: Focused on LHS/RHS activity and swipe back on the
      LHS/RHS of the screen.
Test: Same as above with IME shown
Test: wm presubmit
Flag: EXEMPT bugfix
Change-Id: I7d25e953c27954eb1e36f19bd3ac8452d42f9e7d