Implement latest design on thick brightness slider

Add a DrawableWrapper that can deal with progress bar with rounded
corners. The right (end) bound indicates the level of the progress, so a
full/empty bar has the expected value.

For some reason, start/end in the icons don't work as expected with RTL,
but left/right do.

Test: manual
Bug: 171319433
Change-Id: I291375ac63dfb8f18865c9b3e90c234d32ac9e7d