[pm] get data dir without AndroidPackage

An app's data dir can still exist even if the APK is deleted and
when the corresponding AndroidPackage is null. This CL is to make it
possible to get the data dir path without using the AndroidPackage
object in PackageSetting.

BUG: 288142708
Test: run dumpsys on a package deleted with DELETE_KEEP_DATA and check
"dataDir=" in the output
Test: after reboot the output is still the same
Test: to add a cts test for this in a follow-up CL
Test: atest com.android.server.pm.PackageManagerSettingsDeviceTests

Change-Id: I8d2777af46d8ac2c541e22a2532195776e145a50