InputMethodSubtypeArray: prevent negative count injection

Fixes an issue where negative counts could be injected
via the Parcel constructor. The writeToParcel method
in that case would write data that a subsequent read would
not consume.

Fixes: 277916797
Test: atest InputMethodSubtypeArrayTest
Change-Id: I7e881d82415051179c59bf5df97f8ba0a41e693e