Add unit tests for InputMethodSubtypeSwitchingController

This CL adds unit tests for InputMethodSubtypeSwitchingController
as a ground work to make it aware of
supportsSwitchingToNextInputMethod in a subsequent CL.

This CL never changes existing behavior.

BUG: 12981505
Change-Id: I3b2c46c47c7686b811fa248ad549f20875367425