Move permission-by-UID check earlier

StartPackageManager during boot has a short flurry of binder activity,
which is caused by a per-package call to ActivityManager.getService().
ActivityManager.getService() only returns null at this stage in boot, so
moving up the root/system UID check to occur before getService() saves
some time.

Test: build, boot, verify boot time improvement
Bug: 383159377
Change-Id: I3f873e09f291c385d6844be87f4c211eab657dbe