Exposing TelephonyManager.isIccLockEnabled as a system API.

This was recently removed as part of un-exposing telephony
mainline related APIs (b/146983487), but being added back in R as GMSCore
already uses it (b/151742706).

Test: TH
Bug: 151742706
Bug: 146983487
Change-Id: If4e8e87c7dfbb261ee3abfec2a4774980c24d0c2