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>
build: Set ccache base to build top

This enables ccache sharing between different build tree paths.

Change-Id: I99636bdd2779a6ccf0649f027e35cf67df44b6ae