[bat] use battery charging fg color for chip bg

This helps with contrast for the battery plugged-in animation for the
compose battery. It's always green for now, so we can always use
the foreground color to ensure contrast.

Test: manual; unplug and replug the device
Bug: 399560295
Flag: com.android.settingslib.flags.new_status_bar_icons
Flag: com.android.systemui.status_bar_root_modernization
Change-Id: I7b228c0747a14ddd168bbdd3fa3d25c94a90443e