The Android Designer editor always opens files in the "Design" mode by default. Once you have it open you can switch to "Text" mode and it will retain that setting for that file. However, there's no option to have it default to opening XML files in "Text" mode.
In the new Android Studio 4.1 this option is somewhat changed to :
To access this visit
Android Studio -> Preference from the menu bar search for design and select layout editor and change the mode to your desired one.
Since in the new update, the feature of viewing the code of XML has changed, there are some series of steps that are needed to be followed.
Step 1: Press Ctrl + Alt + S (Basically opening settings)
Step 2:Click on the layout editor and choose the default Editor mode as you wish. In my case I want it to be in split mode (You will be able to see both code and its output at the same screen or window).
Step 3: Click on Apply and OK.
Step 4: Close all the opened XML or the design files . Now after this step , if you open any other file present at your project, you would be realizing that the mode in which the file is opened is at the mode you chose at the Step 2 .