storageproxyd: Use alternate data path if in DSU state

Adds a check for a DSU mode boot in storageproxyd. Changes path handling
so that storageproxyd will not allow opening a file in the root data
path in DSU mode. Instead, storageproxyd creates an "alternate/"
directory in the data directory and the TA must use this directory to
store its backing file.

Re-landing reverted change: Iad68872dc6915f64eaf26cd3c92c04d9071ef169

Test: Boot into DSU and inspect logs for "Cannot open root data file"
Test: Test that TD writes in DSU mode don't corrupt host image storage
when using a compatible storage TA that supports alternate data mode.
Bug: 203719297

Merged-In: I1d07e7c3d15dc1beba2d340181d1b11a7988f869
Change-Id: I1d07e7c3d15dc1beba2d340181d1b11a7988f869