Add SeekBar listener extensions

[bernaferrari]

Two things to notice:

• I was only able to make the test for onProgressChanged, since onStartTrackingTouch and onStopTrackingTouch deal directly with UI and private methods.
• I'm not sure the name from a method is the best one. The original is "onStartTrackingTouch", I made it become "onStartTracking", is this ok?

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[bernaferrari]

I signed it!

[googlebot]

CLAs look good, thanks!