Support multiple app versions across system partitions

The APK with the higher version code will be preferred. This change
skips committing any packages found during scan that have a lower
version code than the package persisted in Settings.

If somehow the newer version cannot be scanned, the older version(s)
will be queued to re-scan, taking the next highest version that does
scan successfully.

As a consequence of this, PackageParser2/PackageCacher was updated
to use the first parent folder of the app being scanned as part of its
cache key. This allows multiple versions to be placed in folders/files
with the same file name, but across partitions. It only takes into
account this first path segment.

Also, a bug was fixed where PackagePartitions wasn't null checking
folders when checking contains.

Bug: 140494776

Test: manual test adding system app to multiple parititions, removing
    either/none/all and rebooting device
Test: atest com.android.server.pm.test.SystemAppScanPriorityTest
Test: atest com.android.overlaytest.remounted

Change-Id: Ie09ccf4b64a0be26d19c9034a68ca4877ca49b81