ServiceManager: Restore basic uid check

Prevent apps from registering services without relying on selinux checks.

Bug: 29431260

Change-Id: I38c6e8bc7f7cba1cbd3568e8fed1ae7ac2054a9b
(cherry picked from commit 2b74d2c1d2a2c1bb6e9c420f7e9b339ba2a95179)
