swift234

[android] Support latest Android NDK r11c

- Update the documentation to reflect that Swift supports (only)
  the latest NDK version. Based on what I've heard from Android
  developers that use the NDK, this is a reasonable requirement.
- The most recent version of the Android NDK no longer includes a
  "4.8" toolchain version. Change the default to "4.9", and update
  the paths in the documentation to match.
- The build script option "--android-ndk-version" was misleading.
  This parameter actually refers to the Android API level. Swift
  currently supports 21 (Android 5.0) and above (although supporting
  lower API levels would be desirable).