[battery][old] use the NewStatusBarIcons object instead of the flag value

The old-new battery icon was relying solely on the newStatusBarIcons()
flag value directly.

The bug is that when status_bar_root_modernization is on but the
new_status_bar_icons flag is off, then users will see an old legacy code
path + old-new battery.

The simplest and safest fix is to simply replace the calls to
newStatusBarIcons() with NewStatusBarIcons.isEnabled. This logically
ensures that users will only see _either_ the old-old battery or the
new-new battery, but never the old-new battery.

All of BatteryMeterView can be removed once the new_status_bar_icons
flag fully rolls out anyway, so a minimal code change is justifiable
here imho

Test: manual
Bug: 314812750
Flag: com.android.settingslib.flags.new_status_bar_icons
Flag: com.android.systemui.status_bar_root_modernization
Change-Id: I30216a54900b0c67bee8c80623a01bb8355e4b38