Support IMEs that have no subtype in PhysicalKeyboardFragment.

With a Framework-side change [1],
InputManager#{get, set}KeyboardLayoutForInputDevice() are now able to
deal with null InputMethodSubtype as a valid input.  With this CL, we no
longer filter out IMEs that do not support subtypes in
PhysicalKeyboardFragment.

 [1]: Ia013784a594ad3beaf30976d047f5ac0fa8185be

Bug: 28182650
Change-Id: I46b9c5b018f08e3eaa4614a0893db0be91652f3c