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
63d0a06799