Consider the following sentence:
I am developing an application to be installed on Android.
This has been a major flaw in Android...
To be clear I am unsure of the usage of "in" vs. "on". When should I use "in" and when to use "on"?
It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, see the FAQ.
Install software on a platform. Find defects in a program.
If you are developing an application to be installed in Android, then it's going to be incorporated into that software and form part of the platform.
If you find a major flaw on Android, you are not talking about Android itself, but something else which is running on that platform (and which works well on other platforms).