chore: fix project build and setup GitHub Actions

[thatfiredev]

This PR should setup GitHub Actions on the project and fix some build errors:

• match compileSdkVersion and targetSdkVersion
• fix type inference errors on ui's C2BPaymentFragment.kt
• use AndroidX's CardView instead of the Support Library
• make non-null asserted call to classLoader

Also notice that when building Pull Requests, GitHub Actions will call gradlew assembleDebug testDebugUnitTest which is much faster and catches obvious bugs.

[realdm]

Changes look good to me.
Please squash your commits before your merge.

[thatfiredev]

@realdm I've squashed the commits. I don't have write permissions on this repo, you'll have to merge it.
You might also need to enable Github Actions on the repo settings after merging.