Expose AndroidPackage#getStorageUuid

The system internally passes around a String, but to mirror the
public ApplicationInfo API, PackageState and related
infrastructure will act on real UUIDs.

These can be converted with StorageManager#convert, although it's
expected that any exposed @SystemApi methods will just take the
UUID as a paramter.

Bug: 254029038

Test: atest AndroidPackageTest

Change-Id: I93e0f04b4d5c93cd534d393057f2c0d7f61418df