Add some tests for TarBackupReader

... and PerformAdbRestoreTask.

Involves some refactoring, splitting readAppManifest() into two methods.

Also a bit of cleanup: make private field actually private and use
static imports for some constants.

Bug: 38090803
Bug: 37619463
Test: runtest -p com.android.server.backup frameworks-services
Change-Id: Ic30a6c5a515da1efb67caaae6eb75f4313797d5c