Fix the Ephemeral guest mode UI bugs

- Update the user-icon size to 40dp to align with add-icon size
- Remove the guestInfo preference and add the infoText as the summary for the guestExit preference and fix UserSettingsTest

Bug: 231592331
Test: Manual test, atest SettingsRoboTests
Change-Id: I2591bc19a7b2381d1842f0b28ab8fa19c15c19b8

tests/robotests/src/com/android/settings/users/UserSettingsTest.java

@@ -185,7 +185,6 @@ public class UserSettingsTest {
         mFragment.mGuestCategory = mock(PreferenceCategory.class);
         mFragment.mGuestResetPreference = mock(Preference.class);
         mFragment.mGuestExitPreference = mock(Preference.class);
-        mFragment.mGuestInfoPreference = mock(Preference.class);
     }
 
     @After