Improve sorting performance by 2.5 times.

This CL replaces List<String> with String[], which prevents from
calling get() and set() multiple times within a loop, in favor of
System.arraycopy().

Scanning a directory with 10K files went down from 1200ms to 450ms.

Bug: 27286016
Change-Id: Id533480934f739905a845cb0e13fe862e361b3db