Invalidate IMMS#mCurrentSubtype when upgrading the current IME.

Whenever the package to which the current IME belongs is
modified, IMMS#mCurrentSubtype needs to be invalidated.
Otherwize, InputMethodManager#getCurrentInputMethodSubtype()
continues to return the previously selected subtype.

One example scenario would be to update metadata of a subtype
in a new version of an IME with keeping the same subtypeId.
Suppose imeSubtypeExtraValue was updated from "ABC" to "XYZ".
In this scenario, the subtype in question retrieved with
IMM#getInputMethodList() then InputMethodInfo#getSubtypeAt()
has imeSubtypeExtraValue "XYZ", while the subtype returned
from InputMethodManager#getCurrentInputMethodSubtype() still
has imeSubtypeExtraValue "ABC".

With this CL, IMMS#mCurrentSubtype will be invalidate when
the active IME package is modified.

Bug: 21310194
Bug: 20708002
Change-Id: Ifbb35034d9c8e477ebb5353d2a470dfc1627bbd5