Refactor OWNERS file.

This makes it easier to read the per-file exeptions and to make
further changes.

Specifically:
- Collapse multiple lines so we only have one per-file line per file.
- Remove per-file owners who are already global owners of all files here.

See https://gerrit.googlesource.com/plugins/find-owners/+/master/src/main/resources/Documentation/syntax.md.

This reverts many of the changes in
b662614c but I believe does not
actually change the effect of it at all.

Test: None.
Change-Id: I1bef6e806c17344c57377a8ab28109fa40e9ecf2