DO NOT MERGE Uses equals to compare the old/new subtypes.

In case of additional subtypes, which are provided by IME app at
runtime, the subtype instances may be re-created but remains the
identical hash codes.
So when an IME provided subtypes {A,B} with current subtype A and
then provides {A,B,C} later, the IME don't expect to be notified
by InputMethodService#onCurrentInputMethodSubtypeChanged.

Bug: 269546663
Test: locally verify the bug.
Change-Id: Ib23e5883af4b99845e80b370c245999fa10d7f57
(cherry picked from commit e4244a4f)