Show IME switcher also when hardware keyboard is connected.

As per discussion with UX team, we start showing IME switcher UI
not only when there are multiple IMEs are enabled but also when
a hardware keyboard is connected.

This CL also simplifies needsToShowImeSwitchOngoingNotification()
by renaming it with shouldShowImeSwitcherLocked() and unifying
related condition checking into that method.

Bug: 19496012
Change-Id: Id44724a2a46190382c283a52ece8edffb740807d