message: Fix `cmake` build error in Android example
  1. Remove `gcc`(https://android.googlesource.com/platform/ndk/+/master/docs/ClangMigration.md)
  2. Change `gnustl_static` to `c++_static` (https://developer.android.com/ndk/guides/cpp-support.html)
diff: @@ -94,7 +94,7 @@ android {
  }
  externalNativeBuild {
  cmake {
  - arguments '-DANDROID_TOOLCHAIN=gcc', '-DANDROID_STL=gnustl_static'
  + arguments '-DANDROID_STL=c++_static'
  }
  }
  }
  