Hide remove user option for main user in multi user settings

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