[Files]: Enables search on Linux

FileEnumerator cannot search in linux volume. To enable this, we
refactor the local search to return a list of promises for the local file system. To keep it consistent, the drive search promise is also placed in a separate method. Linux search happens under two conditions: either we are searching everywhere, or we are searching My files. The reason for the second case, is due to the fact that Linux files are visually nested under My files.

Adds test to make sure that searching from My Files also finds linux files.

Bug: b:268391360
Change-Id: Iaa206414c9904d5c513101ac54f82decd593f15a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4323238


Commit-Queue: Bo Majewski <majewski@chromium.org>
Reviewed-by: Wenbo Jie <wenbojie@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1118502}