Integrate BackupAgent with B&REventLogger

In this CL we:

1) Create the logger when the agent is created.
2) Add getBackupRestoreEventLogger for agent implementations to retrieve
   it to log their metrics.
3) Add a method to IBackupAgent for the framework to query the results
   of the agent's logging once the agent is done with a B&R session.
   This also requires for the results to be Parcelable.

Bug:252760247
Test: atest BackupRestoreEventLoggerTest.java
	atest BackupAgentTest.java

Change-Id: I4b4483574f604aff8350e9ddef685f27458e9091