add sensor graph sample

[proppy]

Sensor graph is a C++ Android sample that read current accelerometer values and draw them using OpenGL.

It demonstrate usage of the following Native C++ API:

• Sensors
• Assets

/cc @ggfan @hak

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s/gl2jni/sensorgraph

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Done.

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@ggfan @hak @claywilkinson @gguuss does someone have the bandwidth to review this?

[ggfan]

just a few comment:

1. I feel the program leaks the shader object memory [vertex and pixel shader object]
2. the native module name could be something else, more relate to sensor rather than gl2jni
3. is java support egl1.4, maybe use it to replace egl1.0, just an comment
   thank you for creating the sample!

[proppy]

@ggfan feel free to comment directly on the code too.

2. the native module name could be something else, more relate to sensor rather than gl2jni

Done.

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1. I feel the program leaks the shader object memory [vertex and pixel shader object]

You mean I should delete them with glDeleteShader after calling glLinkProgram? If hello-gl2 has also this problem, we should probably fix it too.

I remembered that shader objects need be deleted after use. whether deleted it after linked into program, or the time deleting programs. I thought after it is linked into the program, shader object is not need for that program anymore.

You're right I will update the sample to delete the shader after linking

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3. is java support egl1.4, maybe use it to replace egl1.0, just an comment

I'm just using the default GLSurfaceView with a GLES 2.0 context, I'm not sure how to force GLSurfaceView to use egl1.4, do you have an example?

[proppy]

1. I feel the program leaks the shader object memory [vertex and pixel shader object]

@ggfan Done.

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PTAL

[ggfan]

LGTM

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Travis is green, merging.