Added ACTION_BATTERY_LEVEL_CHANGED

Sent when the current battery level changes.

It has EXTRA_EVENTS that carries a list of Bundle instances representing
individual battery level changes with associated extras from
ACTION_BATTERY_CHANGED

Each event has EXTRA_EVENT_TIMESTAMP representing time when it occured.

Test: manual
Bug: 74020080
Change-Id: I993005950299c5298c9111ca51cc7717e1f029de