Forbid updatable: true on vendor APEXes.

Vendor APEXes do not formally support updates outside of the main OTA
process. We should clarify this at build time.

Test: set vendor apex to updatable, observe failure
Test: apex_test.go
Change-Id: Ife8d242a42415ebf00d7dced72c07cda1fde1839
diff --git a/apex/apex.go b/apex/apex.go
index 75cff7d..89b5f21 100644
--- a/apex/apex.go
+++ b/apex/apex.go
@@ -2377,6 +2377,9 @@
 		if a.UsePlatformApis() {
 			ctx.PropertyErrorf("updatable", "updatable APEXes can't use platform APIs")
 		}
+		if a.SocSpecific() || a.DeviceSpecific() {
+			ctx.PropertyErrorf("updatable", "vendor APEXes are not updatable")
+		}
 		a.checkJavaStableSdkVersion(ctx)
 		a.checkClasspathFragments(ctx)
 	}