feat(background-mode): add missing functions

closes #1078

src/plugins/backgroundmode.ts

@@ -1,5 +1,4 @@
-import { Cordova, CordovaFunctionOverride, Plugin } from './plugin';
-
+import { Cordova, Plugin } from './plugin';
 import { Observable } from 'rxjs/Observable';
 
 /**
@@ -124,4 +123,48 @@ export class BackgroundMode {
   })
   static on(event: string): Observable<any> { return; }
 
+  /**
+   * Override the back button on Android to go to background instead of closing the app.
+   */
+  @Cordova({
+    platforms: ['Android'],
+    sync: true
+  })
+  static overrideBackButton(): void {}
+
+  /**
+   * Exclude the app from the recent task list works on Android 5.0+.
+   */
+  @Cordova({
+    platforms: ['Android'],
+    sync: true
+  })
+  static excludeFromTaskList(): void {}
+
+  /**
+   * The method works async instead of isActive() or isEnabled().
+   */
+  @Cordova({
+    platforms: ['Android']
+  })
+  static isScreenOff(): Promise<boolean> { return; }
+
+  /**
+   * Turn screen on
+   */
+  @Cordova({
+    platforms: ['Android'],
+    sync: true
+  })
+  static wakeUp(): void {}
+
+  /**
+   * Turn screen on and show app even locked
+   */
+  @Cordova({
+    platforms: ['Android'],
+    sync: true
+  })
+  static unlock(): void {}
+
 }