Store up to 5 network log batches if needed.

In the normal mode when the DO fetches the logs ASAP, there will still be
no more than one last full batch in memory at once. If the DO is too slow,
or the broadcast queue is too crowded we will store up to 5 of them,
discarding older ones when there are more than 5.

Also the batch gets discarded 5 minutes after it has been retrieved or
another more recent batch has been retrieved. Previously the last batch
would stay in memory until the next one is ready. But it seems
unreasonable for the DO to rely on it since there are no guarantees.
This would probably even save some memory under normal conditions on
average.

Bug: 35753013
Test: cts-tradefed run cts -m CtsDevicePolicyManagerTestCases -t com.android.cts.devicepolicy.DeviceOwnerTest#testNetworkLoggingWithSingleUser
Change-Id: Ib8e91a98103d804375cb0d7423f93175b4b9bcb6