Can now restore a subset of apps from historical dataset

Adds the ability to filter a restore of an historical dataset so that it
only restores certain apps' data regardless of what is actually present
in the dataset.  This is currently only used by the bmgr command-line tool,
for debugging / developer support.

Bug 2021590

Change-Id: I7685e5d609b0f5506f71d70c26410602bb387659