Improve and simplify final polygon drawing

The final step of the smart select animation, drawing the outline of all
the rectangles, was not working properly in cases where the rectangles
would touch or intersect, which we cannot guarantee that they won't.

Instead of the previous implementation, we now rely on Path to create
the outline of the rectangles, making much of the code simpler.

Now the polygon generation should work in the general case, which is one
of the prerequisites in getting the smart select animation to work
properly in multiline scenarios.

Test: manual - test the smart select animation with various rectangle
sets
        (100, 100, 500, 500), (300, 300, 700, 700)
        (100, 100, 500, 500), (500, 100, 900, 500)
        (100, 100, 500, 500), (500, 300, 900, 700)
        (100, 100, 500, 500), (300, 300, 700, 500), (200, 500, 400, 700)

Change-Id: I51dd72e18c5efe36df734aa62ab47d57a5946399