Android: Add build types for easy testing/signing

Currently, if a user has to install a different version of Krita to test on their device. They have to first uninstall the Play Store version (pretty much losing all the configs etc), and then install the app. And this process has to be reversed if the user wishes to move back to the Play Store version. This also limits the testing of Android Nightly where most of the people who install it, have to self sign it.

With this branch, a user can install nightly version and the debug testing builds. And they will be installed parallel to the Krita from Play Store.

Once this gets merged, we'll have to enable the NIGHTLY_BUILD flag on the binary factory for nightly and sign the APKs and maybe put them on krita.org (?). So more users can easily test the nightly.