Add groups to external/icu/OWNERS.

Test: build/make/tools/checkowners.py -c -v external/icu/OWNERS
Change-Id: I5ffe981f7870002ac4784039e6266fd00c98d3a7
diff --git a/OWNERS b/OWNERS
index 94d2383..d24c83c 100644
--- a/OWNERS
+++ b/OWNERS
@@ -1,5 +1,7 @@
+icu-team+reviews@google.com
 mscherer@google.com
 roubert@google.com
 
+android-libcore-team+review@google.com
 jsauer@google.com
 nfuller@google.com