You can do that by editing default template files, which are located in:
...AndroidAndroid Studiopluginsandroidlibemplatesactivities
so for example, default layout file for template of EmptyActivity (note: this one is specific, the other ones have their own res folders) is located in:
...AndroidAndroid Studiopluginsandroidlibemplatesactivitiescommon
oot
eslayoutsimple.xml.ftl
by simply replacing RelativeLayout with LinearLayout in your favourite text editor, you get what you want, but instead of that, I recommend you to make your own template, where you can define literally anything, according to your desires.
Edit: For the BlankActivity and a guide how to edit the other templates see this link:
How to change a BlankActivity template default files and a guide how to do it for any others.
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