DO NOT MERGE: Fixes NPE when preparing app data during init

When deleting an unused static shared library on Q, the user manager was
fetched via mContext.getSystemService. At this time during boot, the
service wasn't registered and so null was returned. This has already
been addressed in R with a move to injecting dependencies in the
PackageManagerService constructor.

Bug: 142083996
Bug: 141413692
Test: manual; remove static dependency on eng Q build and reboot
Exempt-From-Owner-Approval: Already approved in
https://googleplex-android-review.git.corp.google.com/c/platform/frameworks/base/+/9684883/

Change-Id: I8ae4e331d09b4734c54cdc6887b273705dce88b1
Merged-In: I8ae4e331d09b4734c54cdc6887b273705dce88b1