Replace the permission of internal connectivity checks

A number of connectivity checks that protect system-only methods
check for CONNECTIVITY_INTERNAL, but CONNECTIVITY_INTERNAL is a
signature|privileged permission. We should audit the permission
checks, and convert checks that protect code that should not be
called outside the system to a signature permission. So replace
all CONNECTIVITY_INTERNAL to other proper permissions.

Bug: 32963470
Test: atest FrameworksNetTests NetworkPolicyManagerServiceTest
Change-Id: I8f2dd1cd0609056494eaf612d39820e273ae093f