Implement vendor as a synonym of proprietary

The vendor image will have more than just proprietary modules in it
under Treble, so let's stop marking open source code as proprietary just
to move it to vendor.

Bug: 36452052
Bug: 37134596
Test: compare build.ninja before/after, no changes.
Test: Set vendor: true, ensure it works.
Change-Id: I44b0ec7007d0e311bdcbd44b238b1ef2d05cc6ff
diff --git a/android/paths.go b/android/paths.go
index fe639e6..969c753 100644
--- a/android/paths.go
+++ b/android/paths.go
@@ -649,7 +649,7 @@
 	var outPaths []string
 	if ctx.Device() {
 		partition := "system"
-		if ctx.Proprietary() {
+		if ctx.Vendor() {
 			partition = ctx.DeviceConfig().VendorPath()
 		}