Support starting fuse from a block map

Factor out a new function from ApplyFromSdcard that installs a package
from a local path. Inside this function, we start the fuse and choose the
type of data provider depending on the path string. And similar to the
existing logic, we treat the package as a block map if the path starts
with a '@'.

This is part of the effort to install larger than 2GiB packages on ILP32
devices.

Bug: 127071893
Test: Build a 32 bit sailfish and create a 3GiB OTA package. Sideload
the package, uncrypt and install the package from sdcard.

Change-Id: I328ea34fa530731acbce7554bfc3059313ad6ece