VCN: Remove Telephony hidden API usages

This patch removes three hidden API usages for moving
VCN to a mainline module. They are:
- TelephonyManager#getNetworkTypeName(int)
- TelephonyNetworkSpecifier(int)
- SubscriptionManager#isValidSlotIndex

Bug: 288465472
Test: atest FrameworksVcnTest && atest CtsVcnTestCases
Flag: EXEMPT pure refactoring
Change-Id: I09e68006461ac9c5a69eddd27f801f9ed40d15fa