Revert "Accept null subtype in InputMethodSubtypeHandle."

This reverts commit 46ac35d0 [1].

Reason for revert:
This is the part 1 of a series of reverts to unlaunch Bug 25752812,
which aimed to improve UX but did not work well.

See I7a2ed6dd39dcd8207d3d94e12cd01d5d67ba4bb5 for the detailed reason
of revert.

 [1]: Ia013784a594ad3beaf30976d047f5ac0fa8185be

Bug: 66498367
Test: Manually done
Change-Id: Ifd17b53704b3dff75b6ef3be6835f51271859933