Improve performance of storage measurement.

When calculating directory sizes of data living on emulated external
storage, translate the path to use the internal backing data, which
avoids going through the emulation layer.  It carefully retreats to
the original path when it runs into trouble.

Testing with a hierarchy of 10 directories deep and 2 directories
and 10 files wide at each level, this change improves performance
from 5900ms before to 250ms after; over 20 times faster (!).

Bug: 8172425
Change-Id: Ia7365416f091e102bf7345a49f7d7209a22580a9