Clear the focused app after reset adjacent TaskFragments

The TaskFragments were stacked when the device folded. So, the
focused app was occluded and became invisible. In that case,
there was no focused window and ANR happened.

Clear the focused app if the focused app became invisible after
resetting the adjacent TaskFragments.

Bug: 255631222
Test: locally verified by changing display size
Change-Id: I3048824d6df8b63ea9a0f46e9167d9e5fc0da580