Update OWNERS for FileSystemProvider

It is a helper class for DocumentsProvider which is owned by the storage team.

Cherry picked I7355d6de4083f59ebbfd481f415ad70f5af996c3 to master due to b/286408867.

Code was merged via `-s ours` and is missing even though the sha exists on the target branch. Cherry pick is required to bring in the code.

Manually remove the "Merged in" directive to allow for downstream propagation.

BUG: 283962634
Test: n/a
(cherry picked from https://android-review.googlesource.com/q/commit:5a3423ad748b1ffd6af40c70d949ab1dd847fb28)
Change-Id: I1856c3b17385b73a86c71ead49b50fd690083d5e