When dumping connectivity metrics events, print data as well.

Currently the output is not very interesting, looking like:

ConnectivityMetricsEvent(1461591945223, 4, 5): Bundle[mParcelledData.dataSize=340]
ConnectivityMetricsEvent(1461591945289, 0, 2048): android.net.metrics.NetworkEvent@b76cd61
ConnectivityMetricsEvent(1461591945296, 4, 5): Bundle[mParcelledData.dataSize=340]
ConnectivityMetricsEvent(1461591945299, 0, 3072): android.net.metrics.DefaultNetworkEvent@e2f3086
ConnectivityMetricsEvent(1461591945527, 0, 2051): android.net.metrics.ValidationProbeEvent@884ad47

but at least it's better than nothing, and event logger users can
always implement a better toString method on their events.

This is only enabled if --events is passed into the dump.

Bug: 28204408
Change-Id: Ife96b8d54e5e91317d86b622d5efc29724969f63