[MediaProjection] Introduce hidden/visible callback.

Notify client when the captured content becomes hidden, or
at least partially visible. This is only relevant if the user
selected per-app capture, and the captured app becomes visible
or hidden.

Test: atest WmTests:ContentRecorderTests
Test: atest WmTests:WindowContainerTests
Bug: 260083492
API-Coverage-Bug: 260083492
Change-Id: I29240bef2a4d08b9fea45cd7cf07a1a7ca203505