Add test to check that DownloadManager maintains extensions.

HC introduced a regression where the extension of a file was changed
to match the mimetype of the file, whereas GB left the requested file
name alone.  This will be fixed for ICS, and this test should ensure
that no further regressions occur.

Bug: 5196436
Change-Id: I68f6295a2d86022f1cdecb250ff8d5b3ab670019