How can I stop Android Studio Indexing/Scanning/Building altogether at startup?

You can actually cancel the build on start. I find this build to be pointless for a variety of reasons:

• most devs would start the program then connect the phone/open the emulator first
• even though the builds the app this doesn't mean much because when you click build again it starts over...

The way to cancel the build is just simply click on the little red x at the task at the bottom.

When you ready to build then you can just click on run again.

I consider this a bug because it's indeed pointless since it does not make consequent builds any faster nor it is required for the IDE to work unlike what the accepted answer suggests.

However mind that this does not the same as "Indexing" which is Android Studio indeed need. However on a normal startup case you can just cancel the build.

I have an open request for this at Google, we will see if they actually do something with it in the future since I don't see the point of wasting sometimes minutes for a initial build I cannot even use later.

I solved it by just Invalidate and Restart. I experienced this isssue in a loop, indexing non stop without Run or Build anything.

File --> Invalidate Cache/Restart.