Update "Manage PIN" entry point.

- Updates default icon to outlined version
- Makes availability conditional on existence of supervising credential
- Does not disable entry point when the main switch is disabled

Bug: 405159398
Test: atest SupervisionPinManagementScreenTest
Test: atest SupervisionDashboardScreenTest
Flag: android.app.supervision.flags.enable_supervision_settings_screen
Change-Id: I764a6b767019007a93aacf29ecf47677e16cb058