[Wi-Fi] Check default data SIM instead of all active subscriptions for

        'Switch to mobile data automatically' setting

To align with framework design.

Bug: 138956509
Test: CellularFallbackPreferenceControllerTest
      manual test:
          1. Insert feature supported SIM and check UI.
          2. Remove feature supported SIM and check UI.
          3. Turn airplane mode on and check 1. and 2..

Change-Id: If10616c9b47fae490521479c5abc99c5df87f409