ccache: Use system version over prebuilt

If there is a ccache binary on the
host system use it instead of the
AOSP prebuilt.

Change-Id: I4b1e030713be22167a12bd229a4e35eda7736cd2
Signed-off-by: Evan Anderson <evan1124@gmail.com>
Merge remote-tracking branch 'PA-build/kitkat' into kitkat