Grant DCS storage access; better OBB errors.

DCS had been relying on the WRITE_MEDIA_STORAGE permission to access
OBBs on external storage, but that permission has been locked down,
and we need to use the real WRITE_EXTERNAL_STORAGE permission now.

Rework the OBB error reporting flow to bubble exact error codes up
from internals, so that we can return expected CTS error codes.

Test: cts-tradefed run commandAndExit cts-dev -m CtsOsTestCases -t android.os.storage.cts.StorageManagerTest
Bug: 73424392
Change-Id: Iecbc4132745d56ebf081868ad2f9c3efe1e3735f