Do not require the backupAgent manifest property to set some other backup flags

backupAgent is not required for full backup (it is for key-value backup). The current implementation means that apps participating in full backup cannot set the restoreAnyVersion or killAfterRestore flags, which could be relevant for them

Bug: 414373723
Test: Manual inspection of package dump
Test: Manual test of restore to app that tries to set restoreAnyVersion and previously failed
Test: atest PackageParserTest
Flag: EXEMPT bugfix
Change-Id: Ic78b9bd1ded987112b91935ec11975726afaf892