Refactor getBootUserUnchecked() to extract a couple of methods

Bug: 374926694
Change-Id: I5e2dd362e3ed5905182fba6de7f0fdd3edab93f1
Flag: EXEMPT refactor of the code
Test: manually build and deploy in DUT
Test: atest FrameworksMockingServicesTests:com.android.server.pm.UserManagerServiceTest
1 file changed