If the terms of the license to which you agreed when choosing to incorporate someone else's work into your own stipulate that attribution be made on the app description page, that's where you have to make the attribution. If you're not willing to do that then you'll need to find different assets with different licensing terms.
On the Github library question, again you're bound by the developer's chosen licensing terms. You can read the full text of the Apache License 2.0 here:
https://www.apache.org/licenses/LICENSE-2.0.html
This page provides an overview:
https://choosealicense.com/licenses/apache-2.0/
Basically you're free to use the code but
must include the existing license and copyright notice, as well as clearly specify if you've changed any of the contents of the licensed work.
Many apps I use have a 'Third Party Licenses' section in the app's Settings or About screen where they provide the necessary documentation for anything they didn't create themselves. (But again, if flaticons requires attribution to be made 'on the description page on the app store' then you'll have to do that as well.)