Update platform_apis error message

The updated error message is more descriptive about the incompatibility
between platform_apis and sdk_version. Implemented as per the discussion
in b/204447559#17

Test: In build/soong, run go test ./java
Bug: 204447559
Change-Id: I826f0ce47313b337f1754a1b6bc11ce65c837a41
diff --git a/java/app_test.go b/java/app_test.go
index 07439fc..0aae928 100644
--- a/java/app_test.go
+++ b/java/app_test.go
@@ -144,14 +144,14 @@
 		}
 	`)
 
-	testJavaError(t, "platform_apis must be true when sdk_version is empty.", `
+	testJavaError(t, "This module has conflicting settings. sdk_version is empty, which means that this module is build against platform APIs. However platform_apis is not set to true", `
 		android_app {
 			name: "bar",
 			srcs: ["b.java"],
 		}
 	`)
 
-	testJavaError(t, "platform_apis must be false when sdk_version is not empty.", `
+	testJavaError(t, "This module has conflicting settings. sdk_version is not empty, which means this module cannot use platform APIs. However platform_apis is set to true.", `
 		android_app {
 			name: "bar",
 			srcs: ["b.java"],