Updating 'search' icon to latest WVUI/OOUI optimized, reduced path. Also
- amending size of search input and position of icon in input slightly to
align it closer to standard text inputs
- simplify CSS by getting rid of selector which is targeting both, input and
button and applying rules only where needed
- fixing code comments
- increasing icon size to `16px` equivalent `em` to enable user text zooming
preference applied to search icon as well. `16px` is a compromise towards the
old look and feel of the previous icon bringing it closer to standard icon size in
legacy Vector.
This change affects both modern and legacy versions of Vector.
Bug: T266166
Change-Id: Ib4c0c74d3cac30e1893f4c76e56e1197652d41ba
795de73496
