Enable preventing IME startup for non-edit views

This CL introduces the config_preventImeStartupUnlessTextEditor
configuration option, which is disabled by default.
When enabled, this prevents the IMMS from triggering an onStartInput
unless the window receiving focus contains a text editor element
or IME startup was explicitly requested by the client app.

Bug: 199887357
Bug: 37617707
Test: set the config option to true in an overlay
Test: manually on adt3 and another android tv device
Test: atest CtsInputMethodTestCases
Change-Id: Iacb29cfdb3825248ee8c40e610652f697aa2d6da