missing permission check to access multi_sim_data_call settings

this private settings can be accessed from alternative API
SubscriptionManager.getPreferredDataSubscriptionId which requires
READ_PRIVILEGED_PHONE_STATE permission. Add additional permission
check for settings access to avoid any potential security holes.
note: the alernative API from SubscriptionManager is also a hidden
one, without the public alertnative API we have to handle app-compat
properly to avoid breaking apks targeting previous SDKs.

Bug: 172670679
Test: Manual
Change-Id: If527f375da33cc6c30c6513c82bf529209da277a