Android: Port build system to CMake 3.7 capabilities

CMake 3.7 adds support for cross-compiling to Android without a complex
toolchain file.  Port our main CMake build system to work without the
`android.toolchain.cmake` file:

```
$ cmake ../VTK \
    -DCMAKE_SYSTEM_NAME=Android \
    -DCMAKE_ANDROID_NDK=/path/to/ndk \
    -DVTKCompileTools_DIR=/path/to/vtk-compile-tools \
    -DOPENGL_ES_VERSION=3.0 \
    -DVTK_Group_StandAlone=OFF
```