Add support for overrideable boot user id

Adds a SystemProperty override that can override which user the system
boots into by default.

System Property: android.car.systemuser.bootuseroverrideid

BUG: 124779211
BUG: 121197550
Test: atest CarServiceUnitTest
Test: Manually set override value and test to ensure we boot into the
correct user.
Change-Id: I24d54da49e1d78ee4d6fa4647dad3b6c996b4c47
2 files changed