Now that the header collapses at small resolutions
(I89d75843ca7e33e6de93af5d7c22e46b7249c4b7), this commit wires the
search toggle to show the search box when clicked and hides it when the
user clicks outside the search box.
* Adds searchToggle.js to perform handle the toggle behavior of the
searchbox.
* Adds `@padding-horizontal-tabs`, `@size-search-expand` to variables.less
so that these can be used to set the start margin of the search box
(enabling its start edge to match the tab text start edge).
* Modifies screen.less to only apply search max-width when >=
@width-breakpoint-tablet
Bug: T284242
Change-Id: I82563d44967f60aee1cd4d3aa6fb4f405822686b
