Add configurable default setting for assistant componentname

By requiring an explicit default setting, it eliminates all ambiguous
states where a disambiguation would normally be shown. With this
change in place, at no point should a disambiguation be shown.

(Note: this is a fix to Change-Id: Ib32daf that clarifies
the logic if R.string.config_defaultAssistantComponentName
has not been set in a given build.)

Test: manual
Bug: 111603898
Change-Id: Id39fdf700ed4ef08ebbc333b5452223430ac2e48
Merged-In: Ib32dafbd3c6fcbe11186dc8ecab6b09c9b734067