Ensure VcnGatewayConnection#isQuitting never gets unset after being set

This change ensures that the VcnGatewayConnection can never abort a
quitting command; specifically, if a non-quitting disconnect request is
processed after a quitting disconnect request, the isQuitting value MUST
continue to stay set (as true).

Failure to OR the values results in orphaned VcnGatewayConnection(s),
and the VCN network thrashing.

Additionally reduce verbosity of local logs, to ensure it better
captures failures and logWtf(s)

Bug: 192776413
Test: atest FrameworksVcnTests
Original-Change: https://android-review.googlesource.com/1768623
Merged-In: Iec8dae701838794261957609bae4e270eaa9cdc7
Change-Id: Iec8dae701838794261957609bae4e270eaa9cdc7