Cache device owner user id on UserManagerService

UMS needs to check if a user is the DO before removing it, but
calling DPMI at that point would break the LockGuard order.

Test: adb logcat LockGuard *:s # while removing a user
Test: atest UserManagerServiceMockedTest#testGetUserRemovabilityLocked_otherUsers OwnersTest
Test: adb shell dumpsys user | grep "Device owner"
Fixes: 440898996
Bug: 435271558
Flag: android.app.admin.flags.device_owner_for_all

Change-Id: I72e23911f62291a9a72f45f729ff1873493a9115