Lower SMS background importance

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

Flag: com.android.server.am.lower_sms_oom_importance
Fixes: 372511805
Test: manual (`adb shell dumpsys activity lru`, background sms
process should have `prcp+1` oom score.)

Change-Id: Ia4fef6afa57ff822e5122e1716ea05462dd1ed48