[battery] move `isDark` to a provider to avoid recompositions

the bug: `isDark` is a snapshot that changes whenever the dark regions
change. This causes recompositions

the fix: by moving to a provider lambda, we can avoid recompositions

verified manually using printlns to ensure we were only drawing and not
recomposing when the dark regions changed.

Test: manual
Test: existing tests
Bug: 407843100
Flag: com.android.settingslib.flags.new_status_bar_icons
Flag: com.android.systemui.status_bar_root_modernization
Change-Id: I0cde317550f8648bbcba98e8720809f504334f97