Enable lowering IME binding priority

This CL introduces the config_killableInputMethods
configuration option, which is disabled by default.
When enabled, this option lowers the IME binding priority
to below the perceptible level to allow the lowmemorykiller
to reclaim the IME process.

Test: set the config value to true in an overlay
Test: manually on an Android TV device
Test atest CtsInputMethodTestCases
Bug: 199887357

Change-Id: Ib191d964de15bbb84daa8bbeca6f2dca6f96311c