API council suggested edits, part 3

* MbmsException no longer thrown from the common methods, and the error
codes are now containined in MbmsError.
* In order to avoid ANRs while processing download-done broadcasts,
don't perform arbitrary moves when a download is done. Instead, move
into a staging directory and let the app handle it.

Bug: 30981736
Test: testapps
Change-Id: I9416f28b4f24f89af1da6f56a93ea0f0e4ea3878