Ignore unchecked IME show/hide when no root

Both showSoftInputUnchecked() and closeCurrenInput() rely on rootView to
obtain window token. If view root is null, window has already gone away
and IME control had been revoked. Trying to show or hide at this time
would be a no-op.

Bug: 149900693
Test: Manually using the steps mentioned in bug and verify that there is
      no NPE.
Change-Id: I294bb2ec5395d7502a855bafbac672af069e9b4a