Regard timeout as an error in the MtpDocumentsProvider test.

Previously if DocumentsProvider found timeout when terminatnig
RootScanner's background thread, it just output it in error log. Thus
the timeout is not regarded as an error in MtpDocumentsProviderTest, and
it makes flaky PipeManagerTest which runs just after
MtpDocumentsProviderTest.

The CL

 * lets MtpDocumentsProvider throw TimeoutException for timeout.
 * removes redundant resumeRootScanner calls to avoid timeout of
   RootScanner#pause.

Also the CL did cleanup the logic that pauses RootScanner when we don't
find any devices. Previously the logic was in
MtpDocumentsProvider#closeInternal but it is not efficient because we
invokes RootScanner#resume just after
MtpDocumentsProvider#closeInternal. Now the CL moves the logic to
RootScanner so that it can pause itself automatically.

BUG=27638500

Change-Id: Ic11bca67c099cbb0f46679db2f035988045d67d6