Cache the owner name string in UserManagerService

It's read frequently but it won't be changed until a configuration
change. So cache the string.

Bug: 178235824
Bug: 162450085
Test: Manual - switch languages and check 'dumpsys user |grep Owner'
Test: atest UserManagerServiceUserInfoTest
Test: atest UserManagerServiceUserTypeTest
Test: atest UserManagerTest
Change-Id: I337f403e38e9d54f545e773309d8bd1651cbcdf0