allow TOOLCHAIN_VERSION to be overridden

If we could override TOOLCHAIN_VERSION, we would be able to use older NDKs.  I've patched build-droid/build-all.sh to allow for this.  It of course still defaults to 4.7.

Using r7c would look like this, for example:
```
$ TOOLCHAIN_VERSION=4.4.3 ./build-all.sh /foo/bar//android-ndk-r7c
```