Fixes fatal linter errors in android.net.metrics

This patch changes how Event classes are organized:
 - Base IpConnectivityEvent class does not implement Parcelable because
   it cannot be final (has children). It also becomes abstract because
   it is not supposed to be instantiated and logged directly.
 - All children classes becomes final because they are Parcelable.
 - All constructors of all children classes become private, because they
   are supposed to be instantiated with their associated logEvent()
   methods.
 - All instance fields of all children classes become public final.
      Rational: if private, the ConnectivityMetrics app cannot read
      their data.

Bug: 28204408
Change-Id: I1a4689c422230c6ed034307dec54a61daf8a6598