Add hidden intent extra about service availability

Backup has some subsidiary UI needs that are vendor specific,
and policy state is not readily observable from there.  We
therefore inform the sublaunched UI explicitly about the state
via an intent extra in the launch intent (rather than requiring
an API call from the launched UI).

Bug 30126678

Change-Id: I37f9530edbc00eea6c96f2c9fd67e878c07068c9