Quick workaround for a performance regression in IME APIs.

It turns out that the performance of
InputMethodManager#getCurrentInputMethodSubtype() is regressed from ~1ms
to ~20ms when
 - Settings.Secure.SELECTED_INPUT_METHOD_SUBTYPE == -1 and
 - The active IME supports many subtypes (~100)
because we try to find a fallback subtype based on the system locales
every time when IMM#getCurrentInputMethodSubtype() is called.

This could be contributing UI janks because spell checker clients
running in the UI thread indirectly depend on that method.

Fortunatelly the critical path is in
InputMethodUtils#getImplicitlyApplicableSubtypesLockedImpl(), which is
basically a state-less method. We can easily and safely cache its
result by using LocaleList and InputMethod as cache keys.

With this CL the performance basically recovers to the Android M level.

Bug: 28889203
Change-Id: I5ed16c7f14cc18052854f4fd6c9bae8550c332ee