Fixes two BackAnimationController crash from window focus change.

1. Shell side is waiting for animation but got window focus changed,
so mBackNavigationInfo will be reset. Keep previous navigation type
incase shell receive onAnimationCancelled later.

2. Core should preventing from cause window focus change during
startBackNavigation, it's allowed to happen after BackNavigationInfo
return to shell.

Bug: 319293969
Test: presubmit
Change-Id: I9061db055003f661e585fdc647f0bf17be6f9081