A little tweaking of DL Manager tests and adding better debugging output.

Adding more detail to the output logging to help track down issues, fixing some
download completed notification counter flakiness and making reboot test more
robust, and reducing the number of concurrent downloads in testMultipleDownloads()
to 10. After talking with Vasu, this is probably more appropriate as it is
closer to typical usage scenarios, and as a side effect should trim some
time from the test runs. Large numbers of downloads will be left for stress
testing.

Change-Id: Ie337cfe9b8d27299d70553e39c60e241ff3afe66