Be able to override the default NDK for Android kits

Original description:

Be able to override the default NDK for a specific kit, this will make it possible to use one for the majority of Qt versions, but if you need a different one for an older/newer version of Qt then you can override it in the kit settings. This will make it easier to deal with the situation that arose with Qt 5.12.0 where we needed to use NDK r18, but still want to use a different one for the older versions of Qt.

Updated description:
The unofficial hack that users could employ in order to use NDK of choice, is to manually modify Qt Creator's share/qtcreator/android/sdk_definitions.json. The requests to have a UI for that have accumulated.

I suggest having a "Make Default" button next to the NDKs list view, which is in line with what the Kits/CMake lists have. That default NDK version is then used by the Android plugin to override the NDK version for all Qt versions in the sdk_definitions.json. The version substitution should happen while the file is read, so that all parts of the plugin accessing the data are on the same page regarding NDK version (or we might get away with implementing an NDK version getter function that substitutes)

Conscious limitiation: There is no checking whether an installed Qt version is compatible with the set default NDK, the NDK is used for all Kits. Using the 'Make Default" feature hands that responsibility for such things to the user. The button can also unset the default in order to get back to what sdk_definitions.json defines.