[Battery] Ensure we always update the text and content description when

necessary.

When writing some tests for the new dock defend icon, I came across
some existing issues when we don't correctly update the text or content
description. This CL adds tests that would've been failing, and updates
BatteryMeterView to make them pass.

Bug: 255625888
Test: atest BatteryMeterViewTest
Change-Id: I3485a50c6471dd75dce64a3daf414208afa8d25e