Limit user activity notifications to PowerManager from Hub.

This changelist restricts the number and frequency of user activity
notifications through the following changes:
- notifications are only sent when the touch is handled or the hub is
currently showing.
- notifications are rate limited and will be suppressed if arriving
quickly after the last sent one.

Additionally, the call to PowerManager is now made on a background
thread to further reduce contention on the main thread.

Fixes: 403580848
Test: atest GlanceableHubContainerControllerTest#onTouchEvent_touchHandled_notifyUserActivity
Test: atest UserTouchActivityNotifierTest
Flag: com.android.systemui.glanceable_hub_v2
Change-Id: I53b764bd74d7f185aebabcd1d6a5c3b4e7a00ef6