Move per-file OWNERS directives to their directories.

As documented at go/gerrit-owners, per-file directives cannot
contain directory paths. Therefore the current per-file
directives in system/core/OWNERS have no effect.

Test: build/make/tools/checkowners.py system/core/{,libsysutils/src/,include/sysutils/}OWNERS
Change-Id: Ia88b2be42dd50346578bb51dd17fb6136a4591f1