Prevent caching unexpected IME status while showing keyguard in split

While showing keyguard in split, it hides divder bar surface instead of
updating visibility of divider bar view. Updates to make sure
DividerImeController only caches IME status when the divider bar surface
is shown.

Fix: 186466549
Test: manul check password keyguard while in split
Change-Id: I64b406ea3d93cc196fe3304720391b8cb5310111