[DPM] Improvements to the network logs batch finalization mechanism

The full batch will still be available to DPC if there were no
network logs pending.
Added some more debug logging to better investigate the issues.

Test: manual for both cases - pending batch was empty and non-empty,
      with locally decreased timeout
Test: cts-tradefed run cts --module DevicePolicyManager --test com.android.cts.devicepolicy.DeviceOwnerTest#testNetworkLoggingWithSingleUser

Bug: 34245471
Bug: 29748723
Change-Id: Iee229d74d4b0a06025b305a15687f336a0aa337e