Android: Always set CMAKE_ANDROID_NDK_TOOLCHAIN_VERSION

When this variable is not set by the user or toolchain file, set it to
the default selected.  This will be useful for client code that needs to
pass the value to an external tool that needs to find the same toolchain
in the NDK.  Leave it empty for a standalone toolchain.

Suggested-by: Ruslan Baratov <ruslan_baratov@yahoo.com>