Fix rounding of padding in NinePatchDrawable

Bug: 73088142
Test: Test app at https://buganizer.corp.google.com/issues/73088142#comment6
Test: GridWidgetTest.testItemDecorationAndMarginsAndOpticalBounds

In computeBitmapSize, treat padding as a size in scaleFromDensity. This
results in rounding instead of truncating. This matches the way optical
insets are handled. It also matches the padding we would have gotten if
we had scaled at decode time.

Change-Id: I7d3a83ba824b8dc81187ce9b2d9889d4a87f88ac