[SB][Scene] Start/stop tracking battery events separately from NPVC.

Bug: 300484307
Test: enable scene flags -> on lockscreen, verify battery percentage
shows while charging and doesn't show while not charging (unless setting
is enabled)
Test: atest KeyguardStatusBarViewModelTest
KeyguardStatusBarViewControllerTest

Change-Id: I459be774926034887dde4d2121030b2860d4572b