Fix Settings profile selector on HSUM

Currently Settings ProfileSelectorFragment uses isSystem() to identify
the personal user. This change migrates it to use isMain() in line with
HSUM architecture.

Test: atest SettingsRoboTests:com.android.settings.dashboard.profileselector.ProfileSelectFragmentTest
Fixes: 327610266, 327600527, 327585940, 326574736
Change-Id: I064b2fb295b89cffcd556e151fe1bf4484caf384