Implement stop layers

Implement stop layers within SurfaceFlinger. A follow up CL will add an interface for setting a stop layer when creating a mirrored hierarchy.

Bug: 403312802
Flag: com.android.graphics.surfaceflinger.flags.stop_layer
Test: LayerSnapshotTest
Change-Id: I24ab8abbc0715fd165e69c4f7f9b34848d9318b2