Use /var/cache/p2p for P2P_DIR

BUG=chromium:246678
TEST=Unit tests pass, integration test pass

Change-Id: I1fe0529d70b95d0e00bed1fc616c92ddb7bdbcb8
Reviewed-on: https://gerrit.chromium.org/gerrit/57502
Reviewed-by: Chris Sosa <sosa@chromium.org>
Tested-by: David Zeuthen <zeuthen@chromium.org>
Commit-Queue: David Zeuthen <zeuthen@chromium.org>