Move common tar reading parts to separate class.

This moves part of the common code for reading tar files from
PerformAdbRestoreTask and FullRestoreEngine to it's own class.

Change is mostly automated, but with some manual intervention which
doesn't change behaviour.

Bug: 37519282
Test: cts,gts,pts
Change-Id: I669b848e16196ae76fb9e820b786bbe248985e47