Check null for top activity when updating orientation

Ideally if 'r' is non-null, topRunningActivity() should at least
find it. But there may be corner cases such as all activities in
the display are finishing, or other places didn't clear orientation
source correctly for an empty display.

Bug: 266896543
Test: DisplayContentTests
Change-Id: I86a1395e3fcfc3a0338cacd85256c442c5299b1d