Connectivity metrics: log DnsEvents in-band

This patch changes how DnsEvents are logged in IpConnectivityMetrics.
The following changes are made:
 - DnsEventBatch are not logged after 100 queries on the same network
    - this allows to merge DnsEvent and DnsEventBatch into one class
 - DnsEventBatch are not logged after a network disconnect
    - this allows to remove the NetworkCallback
 - DnsEvent are now logged similarly to ConnectStats when statistics are
   flushed, in a direct call from IpConnectivityMetrics into
   NetdEventListenerService, in a direct call from IpConnectivityMetrics
   into NetdEventListenerService.
    - this allows to remove the Parcelable implementation of DnsEvent
 - transports information is added to DnsEvent.

Test: - simplified NetdEventListenerServiceTest covering dns logging
      - updated IpConnectivityEventBuilderTest
      - updated IpConnectivityMetricsTest
      - $ runtest frameworks-net passes
      - manually verified $ adb shell dumpsys connmetrics list proto
Bug: 34901696

Change-Id: Ia4b33fd4212741152662a2adbb0533bd1b4902ee