Create a default package name for device level events.

Device level event like DEVICE_SHUTDOWN does not have package name,
but some user code always expect a non-null packageName for every event.
Create a default packageName for these device level events.

Change-Id: Iee49794bdc6f5cb7b30d50f7421ac0de2a33c858
Fix: b/123429960
Test: NA.