Take supportsSwitchingToNextInputMethod into considertaion

With this CL, InputMethodManager#switchToNextInputMethod starts
behaving as if there are two rotation groups: one is for the new
input methods that are declared with
supportsSwitchingToNextInputMethod set to true to indicate they
have some language switching UI, and the other is for the other
input methods to preserve the existing behavior.

In addition to the above change, this CL also fixes the behavior
of InputMethodManager#shouldOfferSwitchingToNextInputMethod()
so as to return true if and only if the former rotation group
consists of two or more input methods, as originally designed.

BUG: 12981505
Change-Id: I84291fd4a7d6192b3bd0c366c49586e79135584f