[flexiglass] Adjust the battery estimate text color based on the theme.

This is conditional on the `notification_shade_blur` flag, as the shade
color prior to that flag is a constant dark color.

Fix: 414751194
Test: Manually tested by opening the shade on both light and dark modes,
 and observing that the estimate text color changed appropriately.
Flag: com.android.systemui.scene_container
Flag: com.android.systemui.notification_shade_blur
Change-Id: I90a0b702089b20297117440fb6820e40deff6152