Make additional IME Subtype ID persistent.

It turns out that IME subtypes specified to
InputMethodManager#setAdditionalInputMethodSubtypes() are stored in the
presistent storate without subtype IDs.  As a result, when the system is
rebooted, the system would no longer consider those additional subtypes
as enabled due to subtype ID mismatch, until the IME re-adds those
additional subtypes again with the original subtype IDs.

Bug: 28104337
Change-Id: I1445213e0b83d76631a839b974ec1ab9b28ad7d2