fix(hmr): check for hot update should not create new file

Currently the check for hot update creates a new file in case it does not exist (as the method from tns-core-modules is doing this). This is a problem when trying to install `.ipa` on device and the `.ipa` file contains JavaScript files with HMR enabled.
This may happen in case you run `tns run ios` on device and after command finishes the execution open the project in Xcode and deploy the app from there or uninstall it from device and install the produced `.ipa` manually. The problem in the mentioned scenarios is that the JavaScript file cannot write files in the directory where the `.ipa` is installed.
When `tns run ios` is executed, it livesyncs the files in a different location, so the HMR can create the files there.

To fix the issue check if the hmr file exist before reading its content.

Author: rosen-vladimirov

Diff for: hmr/hmr-update.ts

@@ -1,10 +1,13 @@
 import * as hot from "../hot";
-import { knownFolders } from "tns-core-modules/file-system";
+import { knownFolders, path, File } from "tns-core-modules/file-system";
 
 declare const __webpack_require__: any;
 
 export function hmrUpdate() {
-    const applicationFiles = knownFolders.currentApp();
+    const currentAppFolder = knownFolders.currentApp();
     const latestHash = __webpack_require__["h"]();
-    return hot(latestHash, filename => applicationFiles.getFile(filename));
+    return hot(latestHash, filename => {
+        const fullFilePath = path.join(currentAppFolder.path, filename);
+        return File.exists(fullFilePath) ? currentAppFolder.getFile(filename) : null;
+    });
 }

Diff for: hot.js

@@ -147,6 +147,10 @@ function update(latestHash, options) {
 
 function getNextHash(hash, getFileContent) {
     const file = getFileContent(`${hash}.hot-update.json`);
+    if (!file) {
+        return Promise.resolve(hash);
+    }
+
     return file.readText().then(hotUpdateContent => {
         if (hotUpdateContent) {
             const manifest = JSON.parse(hotUpdateContent);