Update Android NDK to r22.
Fixes #329 (closed)
The platforms and sysroot directories have been removed in NDK r22.These directories were merged and relocated into the toolchain during r19. The location of these contents should not be relevant to anyone, including build systems, since the toolchain handles them implicitly. It is not necessary anymore to explicitly set the sysroot include and libs directories because Clang will automatically select the sysroot.