Update OpenImeWindowFromFixedOrientationAppTest (2nd)

To catch Bug 230733128 that the size of TaskSnapshotStartingWindow
surface wrongly rendered by not applying fixed-rotation when the
snapshot included IME,

Add snapshotStartingWindowLayerCoversExactlyOnApp to verify the
size of snapshotRegion should cover appVisibleRegion exactly
in animation.

Bug: 232242249
Test: atest FlickerTest:OpenImeWindowFromFixedOrientationAppTest
Change-Id: I709d8899381126efff82a43a0d39d3342079f122