Enable the clear credentials buttons when wifi keys are installed.

This patch updates the ResetCredentialsPreferenceController to check the
WIFI keystore namespace if called by the primary user.

Test: Install a WIFI certificate or key and watch the
      "Clear credentials" button become enabled in the credential
      storage dialog of Settings.
Bug: 189601008
Change-Id: I69828b64a7e3c707c27b4582d64ff0ddb863a4ff