Added isMain() user check in UserDetailsSettings.initialize and
UserDetailsSettings.canDeleteUser to hide and restrict
delete user option for main user in multi user settings.
Added unit tests for the same.
Bug: 260200162
Test: Manual test on headless to check remove user option
is not visible for the main user.
Test: Run robo tests with this command:
make -j64 RunSettingsRoboTests ROBOTEST_FILTER="com.android.settings.users.UserDetailsSettingsTest"
Change-Id: Ia455611a3d0b1e0945f61f00658425b7bf6c2dae