Create new APIs for overriding APNs

Create new APIs in TelephonyManager for DevicePolicyManager to add
override APNs, and use them in DevicePolicyManagerService instead of
directly accessing the APN database.

Bug: 146834818
Test: cts-tradefed run cts --module DevicePolicyManager --test
com.android.cts.devicepolicy.DeviceOwnerTest#testOverrideApn
Test: atest TelephonyManagerTest

Change-Id: I6c6214424169c05fd452db71a8c6593f8473b9d4
Merged-In: I6c6214424169c05fd452db71a8c6593f8473b9d4