Add BootUserOverrideId system property

This property can be used by OEM in cars to determine the default user
id to log into on initial boot.

BUG: 124779211
BUG: 121197550
Test: atest CarServiceUnitTest
Test: Manually set override value and test to ensure we boot into the
correct user.

Change-Id: I5c097f0ee42514369ebe68f9a4eada9ad563d152