[UUM-20370] Update how to get FrameLayout in addControlsToUnityFrame() in MainUnityActivity.java

[VeryHappyYoung]

Purpose of this PR

Jira link: https://jira.unity3d.com/browse/UUM-20370

\ The issue: Before introducing GameActivity for Android in 2023.1, the way you get FrameLayout was updated.

In 2022.2, UnityPlayer extends FrameLayout.

In 2023.1 UnityPlayer doesn't extend FrameLayout. Instead, it offers getFrameLayout() API

\ \ Solution: Use mUnityPlayer.getFrameLayout() instead of mUnityPlayer

\ Note: Make sure that Activity is selected and GameActivity is not selected as the Application Entry Point in Player Settings (Android)

[VeryHappyYoung]

Yes, you are correct. I wanted you to check if everything is all right. Thank you so much!