[SB] Invalidate battery drawable on shield change; dump more info.

It seems other aspects of the battery drawable like `charging` would
invalidate the drawable directly, whereas `displayShield` wasn't. This
adds that invalidation. I'm not sure if that was what was causing the
bug, but it couldn't hurt.

This also adds more info to the `BatteryController` dump so we can see
who's registered as listening for events and the current state of the
battery drawables. See bug for sample dump.

Bug: 286998719
Test: `adb shell am broadcast -a com.android.systemui.demo -e command
battery -e defender true` -> shield shown
Test: `adb shell am broadcast -a com.android.systemui.demo -e command
battery -e defender false` -> shield hidden
Test: `adb shell dumpsys activity service
com.android.systemui/.SystemUIService BatteryController` -> additional
info is dumped
Test: atest BatteryMeterViewControllerTest
BatteryControllerImplGoogleTest

Change-Id: Idef1e6a883dd09012671ffc19520e8c3664f7dd2