Fix react-native link issue when using multiple manifests

Summary:
`react-native link` often fails due to the wrong manifest being used when you use a debug manifest. `findManifest` returns `debug/AndroidManifest.xml` instead of `main/AndroidManifest.xml`. And the debug manifest usually does not have the package name defined so `projectConfigAndroid` throws a cryptic "Cannot read property 'replace' of undefined" error.

This fixes the issue by throwing a more user friendly error and providing a `manifestPath` userConfig.

This is mostly based on comments to #10050.
Closes https://github.com/facebook/react-native/pull/13373

Differential Revision: D4945690

Pulled By: shergin

fbshipit-source-id: b177f916fd4799c873d2515c18cbb87bef3203f0

local-cli/core/__fixtures__/android.js

@@ -20,6 +20,25 @@ exports.valid = {
   },
 };
 
+exports.userConfigManifest = {
+  src: {
+    main: {
+      'AndroidManifest.xml': manifest,
+      com: {
+        some: {
+          example: {
+            'Main.java': mainJavaClass,
+            'ReactPackage.java': fs.readFileSync(path.join(__dirname, './files/ReactPackage.java')),
+          },
+        },
+      },
+    },
+    debug: {
+      'AndroidManifest.xml': fs.readFileSync(path.join(__dirname, './files/AndroidManifest-debug.xml')),
+    },
+  },
+};
+
 exports.corrupted = {
   src: {
     'AndroidManifest.xml': manifest,