Don't persist description res ids as they are subject to change.

Instead persist the resource name and update the javadoc to suggest
that apps should re-acquire the lease if the description resource
changes.

Bug: 150829640
Test: atest --test-mapping apex/blobstore
Change-Id: Ibeabffe7de046ed74cbbcaed6c69c9d25028799d
5 files changed