BatteryController: Only listen to UEvents from power_supply changes

Previously, BatteryController would listen to all UEvents generated for
the input device's path. However, it should only listen to changes from
the power_supply subsystem, which correspond to battery updates.

Bug: 243005009
Test: atest FrameworkServicesTests
Change-Id: I34e7dfce6eff40ca5c4e39a1de8893e67c1513e1