charger: make libhealthd_charger_ui vendor_available.

This is a prerequisite to move charger to vendor partition.

Because vendor domains can't read ro.charger.*, they are all assumed
to be the default value. They aren't set in the ecosystem anyway,
so it is time to deprecate them.

Test: builds
Bug: 203246116
Change-Id: I0b17e62ab3ac5a89df091333fd9f24e33bdfd49a