Remove use of locks in VcnGatewayConnection.

This CL updates VcnGatewayConnection to not use locking for accesses to
its TelephonySubscriptionSnapshot field. Instead, it will verify that
the caller is running on the VCN-Looper. Because VcnGatewayConnection
also runs on the VCN-Looper, this check guarantees that accesses to that
field are safe even when not explicitly in a Handler event.

Bug: 177364490
Test: atest FrameworksVcnTests
Change-Id: I128a894cead739b88a77e37f4d829d33f6bfb26a