Add dpm force-network-logs command to force network logs retrieval.

This is useful because logs are batched every 1.5-2hrs if there are
not enough logs to fill up a batch. The command is throttled at
10 seconds as a spam prevention measure.

Bug: 62251154
Test: in adb shell run dpm force-network-logs. Observe a new batch
being created in the phone directory
/storage/emulated/0/Android/data/com.afwsamples.testdpc/files/. Also
observe a fresh batch is being displayed on the phone in the TestDPC
app, under "Retrieve network logs".
Change-Id: I5ff9d5c78497ea81533b5248816b4d6e160d338f