Change naming of constants in test.env.paths. (FACTORY_DIR)

This change the naming of constants in test.env.paths for directories
/usr/local/factory/*, according to go/GoofyFHS.

Changes:
* FACTORY_PATH -> FACTORY_DIR
* FACTORY_PACKAGE_PATH -> FACTORY_PYTHON_PACKAGE_DIR
* FIRMWARE_UPDATER_PATH -> FACTORY_FIRMWARE_UPDATER_PATH

BUG=None
TEST=make test

Change-Id: I9b505c788d0f75d2b930c40cc4e7b16aad31260e
Reviewed-on: https://chromium-review.googlesource.com/518604
Commit-Ready: Pi-Hsun Shih <pihsun@chromium.org>
Tested-by: Pi-Hsun Shih <pihsun@chromium.org>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
diff --git a/py/toolkit/installer.py b/py/toolkit/installer.py
index 1fbfa36..76361d0 100755
--- a/py/toolkit/installer.py
+++ b/py/toolkit/installer.py
@@ -476,7 +476,7 @@
 
   args = parser.parse_args()
 
-  src_root = paths.FACTORY_PATH
+  src_root = paths.FACTORY_DIR
   for _ in xrange(3):
     src_root = os.path.dirname(src_root)