Rename IME Snapshot to IME Screenshot

Previously we were using both terms in parallel, which was slightly
confusing. This unifies the references into IME Screnshot, as it is
a better representation of its meaning.

Flag: EXEMPT cleanup
Bug: 281029564
Test: atest DefaultImeVisibilityApplierTest
  ImeVisibilityStateComputerTest
  DisplayContentTests
  ZOrderingTests
  StartingSurfaceDrawerTests#testRemoveTaskSnapshotWithImeSurfaceWhenOnImeDrawn
  CloseImeOnDismissPopupDialogTest
  ShowImeOnAppStartWhenLaunchingAppFromFixedOrientationTest
  ShowImeOnUnlockScreenTest
  ShowImeWhileDismissingThemedPopupDialogTest
Change-Id: I9f75b9db0a8e8555b8bbd63ecac08601930a5390