Don't inject content picker on pages by default

[pes10k]

At least from doing some pagegraph tests, i see we’re loading ./components/brave_extension/extension/brave_extension/content_element_picker.ts [1] on every page load (along with the source map).

Its not enormous but its not nothing. Maybe a small perf benefit to remove that? (since enabling the element picker is a very very rare thing)

Like @atuchin-m suggested on Slack, would be best to only load/inject this when the user asks for the element picker

1: https://github.com/brave/brave-core/blob/5718b3ac479e473a049ae346577709e90fb2ae3c/components/brave_extension/extension/brave_extension/content_element_picker.ts