BUILDING.md: Update/simplify Android build instr. Use the android.toolchain.cmake toolchain file in the NDK (v13b or later), since this toolchain file generally takes care of setting the approprate compiler flags and dealing with the differences between GCC and Clang. Our custom Android build procedure did not work with Clang-based NDK toolchains, which meant that it could not be made to work with NDK v18b or later. Fixes #309