Refactor OWNERS files in platform/build/soong.

Pulls out all of the per-file rules into their relevant directories
so platform/build/soong:/OWNERS can be included everywhere we need
the list of build system owners.

Test: treehugger
Bug: 170407947
Change-Id: I5a4b2d252bd9b3c280cd9954cf0e65ac21153ede
diff --git a/rust/config/OWNERS b/rust/config/OWNERS
new file mode 100644
index 0000000..dfff873
--- /dev/null
+++ b/rust/config/OWNERS
@@ -0,0 +1,2 @@
+per-file global.go = srhines@google.com, chh@google.com, pirama@google.com, yikong@google.com
+