PersistentDataBlockService waits for MainUser

Currently, the PersistentDataBlockService fetches the mAllowedUid too
early for Headless System User Devices (HSUM) to work. On HSUM, the FRP
user (i.e. the MainUser) is created later, so PDBS must wait for then
before trying to find that user's allowed package.

Bug: 262438904

Test: manual: add account+PIN; factory reset into HSUM; SUW demands credentials post reboot
Test: atest FrameworksServicesTests:SyntheticPasswordTests FrameworksServicesTests:WeaverBasedSyntheticPasswordTests
Test: atest CtsPackageInstallTestCases:android.packageinstaller.install.cts
Change-Id: I4b31ba013fcfa36200402e5f57460aecb8325243