fix downloadmanager tests. add few more. fix some. isolate flaky ones

the following work
  frameworks/base/core/tests/coretests/src/android/app/DownloadManagerFunctionalTest.java
    ideally, most of the tests in this should be moved to cts testsuite.
    will require reformatting of comments
    and probably removal of one or two tests using hidden public API.

  packages/providers/DownloadProvider/tests/src/com/android/providers/downloads/DownloadManagerFunctionalTest.java
  packages/providers/DownloadProvider/tests/src/com/android/providers/downloads/PublicApiFunctionalTest.java
  packages/providers/DownloadProvider/tests/src/com/android/providers/downloads/ThreadingTest.java
  packages/providers/DownloadProvider/tests/public_api_access/src/com/android/providers/downloads/public_api_access_tests/PublicApiAccessTest.java
  packages/providers/DownloadProvider/tests/permission/src/com/android/providers/downloads/permission/tests/DownloadProviderPermissionsTest.java

the following need work
  frameworks/base/core/tests/coretests/src/android/app/DownloadManagerStressTest.java
 some of these tests fail in a flaky way. they need work

Change-Id: Ib64645005a8f5faa32e0b9a4c5cacfe6e6ae3063