[SB] Use consistent color in UnifiedBattery

The current implementation can result
in a different color for the battery cap
if a glyph is present and the battery isn't
full.

Test: Manual
Test: BatteryScreenshotTest
Bug: 431251258
Flag: com.android.settingslib.flags.new_status_bar_icons
Change-Id: I9a32828481164b92a9e52c90a55a98f3d4be4e7c