Use the FirefoxOS's NDK instead of downloading it.

Use the NDK that is provided by the FirefoxOS build (cloned from
git://android.git.linaro.org/platform/prebuilt) rather than downloading
it from
http://dl.google.com/android/ndk/android-ndk-r8-linux-x86.tar.bz2

The directories' layout is a bit different between both, so some minor
adjustments to cmake/toolchain/android.toolchain.cmake are necessary.
The default behavior is not changed.