Set the mAccountName as null while device has no any account

It doesn't clean the mAccountName buffer while device has no any account. And it will be put into
the Intent and pass to MeCard when user click the ghost boy avatar. It does not make sense and
modify code to fixed it.

Fixes: 129265427
Test: make RunSettingsRoboTests -j56 ROBOTEST_FILTER=com.android.settings.accounts
      make RunSettingsRoboTests -j56 ROBOTEST_FILTER=com.android.settings.core
      make RunSettingsRoboTests -j56 ROBOTEST_FILTER=com.android.settings.dashboard

Change-Id: Ib8cfab2cb416515018f335b21ef49a95699a1597