screenOrientation.unlock() doesn't respect the current orientation on iOS devices

### Expected behaviour

The device should keep the current screen orientation — the screen was previously locked — after the screen is unlocked.

### Actual behaviour

The screen rotates to the previous orientation when the screen was locked.

### Steps to reproduce it

- The device is in portrait orientation.
- The user does action that locks screen in landscape orientation.
- The user rotates the device to watch screen in landscape mode.
- The user finish the action by which screen was locked.
- Screen is unlocked and it rotates to portrait (instead keep landscape).

### It fails on

- [X] iOS devices

### It works on

- [X] Android devices

### OS versions tested
- iOS 9.3.5
- iOS 11.4.1
- Android 7.0

### Code example

```JS
// player is instance of plyr.io
this.player.on('enterfullscreen', () => {
this.statusBar.hide();
this.screenOrientation.lock(this.screenOrientation.ORIENTATIONS.LANDSCAPE);
});
this.player.on('exitfullscreen', () => {
this.screenOrientation.unlock();
this.statusBar.show();
});
```

### Build system

**Android**
```bash
Ionic:

ionic (Ionic CLI) : 4.0.5
Ionic Framework : ionic-angular 3.9.2
@ionic/app-scripts : 3.2.0

Cordova:

cordova (Cordova CLI) : 8.0.0
Cordova Platforms : android 7.1.2

System:

Android SDK Tools : 26.1.1
NodeJS : v8.11.3
npm : 6.4.1
OS : Linux 4.4

```

**iOS**
```bash
Ionic:

ionic (Ionic CLI) : 4.2.1
Ionic Framework : ionic-angular 3.9.2
@ionic/app-scripts : 3.2.0

Cordova:

cordova (Cordova CLI) : 8.0.0
Cordova Platforms : ios 4.5.5
Cordova Plugins : cordova-plugin-ionic-keyboard 2.1.2, cordova-plugin-ionic-webview 1.2.1, (and 16 other plugins)

System:

ios-deploy : 2.0.0
ios-sim : 7.0.0
NodeJS : v6.10.1
npm : 3.10.10
OS : macOS High Sierra
Xcode : Xcode 10.0 Build version 10A255
```

Start by tracing the iOS implementation of screenOrientation.unlock() and the lock/unlock calls shown in the JavaScript example. Reproduce the listed portrait-to-landscape flow on iOS and compare it with Android behavior; done means unlocking preserves the device's current landscape orientation rather than restoring portrait.

Written by the indexing model from the issue text.