Adapt AndroidConfig::availableNdkPlatforms to new Ndk folder structure

AndroidConfig::availableNdkPlatforms scans the folder structure under <ndk>/platforms/ in to determine the supported android platforms. Until Ndk v21, the folder is present, but not from v22 on.

If there is no other way to get these platform versions than scanning folders, we will have to additionally iterate through <ndk>/toolchains/llvm/prebuilt/<host-arch>/sysroot/usr/lib/<target-arch> in order to determine the available platforms (now also specifically for the target architecture).