Add custom NDK path to the kit settings

Qt Creator has a global SDK and NDK path for Android. This is not good when using different Qt versions that rely on different NDK. That's why it's important to have a custom NDK path in each Android kit settings that can be enabled when needed.

UPDATE:

The current implementation of Qt Creator doesn't allow adding this option (custom NDK) to the kit, it would require lots of changes not to the Android plugin only. For that reason, instead of adding this to the project's kit settings, the user could add custom NDKs to the global settings and toolchains and debuggers would be detected automatically, which then could be used to create custom kits.