Add OWNERS in system/vold

* Owners are selected from top CL approvals or owners.
  They will be suggested to review/approve future CLs.
* OWNERS files are recognized by the new find-owners plugin, see .md files in
  https://gerrit.googlesource.com/plugins/find-owners/+/master/src/main/resources/Documentation/

Test: build/make/tools/checkowners.py -c -v OWNERS
Change-Id: I4d0431105b55e88be258800a7fd45a42deb9bb60

OWNERS

@@ -0,0 +1,3 @@
+jsharkey@android.com
+paulcrowley@google.com
+paullawrence@google.com