[iOS] Refine spacing of "< Back" button

[iamthomasbishop]

Note: This issue is iOS-only.

While inspecting some bottom sheet refinements, I noticed that the spacing of the "< Back" button on iOS could be refined slightly to align with the system-default. Here is a visual:

In other words, we should:

• Remove ~7px of spacing on the left
• Add ~3px of spacing between the icon and text label

It's also worth noting that we're not using the system < (chevron) icon, so if we have that available, perhaps we can swap that in here? I wouldn't consider that a blocker because the icon is almost identical other than the rounded corners, so I think this icon is fine as long as we're matching the spacing.

// cc @lukewalczak is this something you would be able to take care of?