Also rename/change highestAvailablePlatform to highestAvailableSdk.
This fixes a bug reported on irc, where the lists for sdk and ndk
were different and the highestAvailablePlatform was not available
as in the sdk.

Change-Id: I17fec9e8ce1913e933ddcf8eaa21d6bb6e14c5be
Reviewed-by: BogDan Vatra <bogdan@kde.org>