Android: More fixes + improvements to README, and support for $ANDROID_NDK

cjdroid-build.sh: The $ANDROID_NDK variable is now used to locate the Android NDK if the user hasn't configured a custom location using $NDK. The Android NDK packages provided by some distros set $ANDROID_NDK to its installation directory, meaning users of these packages won't require any intervention to get the script to use their system's install.

README.md: More detail was added to some areas that needed it, the debugging step was pulled out and into its own section, some typos were fixed, sections better organized, formatting improved and some info changed to reflect the updated build script (above).