Lower IME background importance

The keyboard app is currently bound with the BIND_NOT_VISIBLE flag
by system. This keeps it in the oom score 200 slot, which competes
with higher priority processes like foreground services. Binding
with BIND_ALMOT_PERCEPTIBLE will lower its background importance to
201, keeping it warm in memory for snappier UX but allows it to
die in favor of process doing more important work.

Flag: android.view.inputmethod.lower_ime_oom_importance
Fixes: 372511805
Test: manual (`adb shell dumpsys activity lru`, background ime
process should have `prcp+1` oom score.)
Change-Id: Ie94ec1bcac212da3fad06186a73502bf01b68def

Change-Id: I41398340eea4d4694f69ccd9abc9e344eee444ef