ui: Fix 1ns event rendering

The previous logic to round resolution up to the nearest multiple of
the resolution was flawed, and made 1ns long slices render as 2ns long
blocks.

This CL changes the logic to work properly for 1ns slices, while
maintaining the same rounding logic to avoid rendering artifacts.

```
Before: ((dur / res) + 1) * res
After: ((dur + (res - 1)) / res) * res
```

Change-Id: Ic23b956cfc896c7d4de42d9d3f2878d80aa62a89