Allow private (& private+Tor) search engines to be changed

[tildelowengrimm]

Currently, private windows have an immutable search engine. That's silly.

[simonhong]

Search engine provider works like below

• Private window (non-tor)
  • DDG is used when it's option is on in private newtab
  • Normal windows's provider is used when ddg option is off
• Private tor window
  • Initially it is to DDG
  • User can change search engine and it is persisted (Merged but not yet released)
  • Qwant is initially set when region is german or France. Otherwise, DDG is initially set (Consider Qwant search provider as a initial provider in tor window brave-core#647)

[simonhong]

@riastradh-brave I think this issue can be closed?

[rebron]

Fixed in RC builds. Reopen if it's not.