Remove taskbar window from WindowManager in TaskbarManager.destroy()

When task bar crashes (from anything), task bar window is not cleaned up properly. This is because destroyExistingTaskbar() only removes the root layout from the window when we are no longer rendering task bar (one use case of that is folding the device). We need to clean it up in onDestroy() also.

Fixes: 319105323
Test: adb shell am crash com.android.systemui and make sure there is only one task bar window
Change-Id: Ia9c808e903422707bf4c270b2631fc913dde65d9