Update scrim if insets are added later.

If insets are set after the activity requests the foreground scrim to be
shown, the scrim will not be correctly added. This is an issue for
fallback recents which requests the foreground scrim right after
initialization.

To fix this, we save the request to show the scrim in a local variable
and check this on inset updates to determine whether we should show the
scrim or not.

Bug: 131853975
Test: Fallback recents now has foreground scrim added correctly
Change-Id: I15a19a3184bbc97a20fbcbba2106fd7221410df0