Fix shared-storage full backup

The special shared-storage step was mistakenly writing its data directly
to the USB output pipe rather than to the proper stacked data handling
chain that applies compression and encryption.  Fix this by getting rid
of the custom handling of the shared-storage data, instead folding it
into the normal data handling flow [with a small amount of additional
management because e.g. it doesn't need a "manifest" pseudofile in the
archive stream].

Fixes bug 5897791

Change-Id: I3995b07963334d2f8cce49b247c87d3d3ff93bed