Fix #test_getFreeSpace flakiness

Using Process and "df" for obtaining the amount
of used, free and total blocks was flaky on
some devices (the amount of free space was sometimes
changing  by page or two, for unknown reason).

Bug: 30587461
Test: org.apache.harmony.luni.tests.java.io.UnixFileTest
Change-Id: I49cd516ba63cf645fad4a7a88dd5932ea607bd27
(cherry picked from commit 7ade2296)