Make hidden subscription apis as system

Make following APIs as system

SubscriptionInfo;->isGroupDisabled
TelephonyManager;->isModemEnabledForSlot
TelephonyManager;->setOpportunisticNetworkState

Add a new public API
SubscriptionManager;->getActiveAndHiddenSubscriptionInfoList

As they need to be accessed by system application to configure
the opportunistic subscripion info

Bug: 141122805
Test: verified that CBRS app is still working fine
Merged-In: Idf1520911272ba4b7c1f15671d6221d99ba0ab37
Change-Id: Idf1520911272ba4b7c1f15671d6221d99ba0ab37