428995 Android: enable GMic

This commit adds a completely optional step to build GMic as a plugin for Android. It also adds a workaround to ensure that its project can pick CMAKE_FIND_ROOT_PATH up.

BUG: 428995

CCMAIL: kimageshop@kde.org

Test Plan

Build Krita using androidbuild.sh.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.