Update battery saver colors to make them more obvious

Use colorError for the plus symbol (when in status bar) and always draw
the fill as colorError when below 15%

Test: turn on systemUI demo mode
      adb shell am broadcast -a "com.android.systemui.demo" -e command battery -e plugged false
      adb shell am broadcast -a "com.android.systemui.demo" -e command battery -e powersave true
      # verify that the plus sign is red
      adb shell am broadcast -a "com.android.systemui.demo" -e command battery -e level 15
      # verify that the fill color is red and the plus stays red

Change-Id: I9898c3d56372bad822aeff021a470958ed3908a6
Fixes: 71873178