Use mainline beta flag for NetworkAgent event queueing

Add `queue_network_agent_events_in_system_server` as a mainline beta
flag. This flag replaces the previous check for
`mQueueNetworkAgentEventsInSystemServer`, allowing the feature to be
controlled and enabled on all Android versions via mainline beta program.

Flag: com.android.tethering.mainline.beta.queue_network_agent_events_in_system_server
Test: atest ConnectivityCoverageTests --update-device \
      --test-filter '.*NetworkAgent.*'
Test: atest ConnectivityCoverageTests --update-device \
      --test-filter '.*Connectivity.*'
Fix: 428098184
Change-Id: I96b0443bd4c55c28d482349738bbdc4798f0c7f9
6 files changed