Density-dependent painting in PointerLocationView

Some of the details around pointers that are drawn in
PointerLocationView are very small, such as circles with radii of 3
pixels. Since screen densities have increased, these are really hard to
see, so treat these hardcoded values as DP intead of pixels so they
appear similar in screens of different densities.

Bug: 215378937
Test: manual
Change-Id: Iad59095ba98761939c6c56c13a426cb447f88713