Add playground support for datastore

Adds a new settings.gradle file under datastore, which allows external
developers to contribute to DataStore using the GH workflow.

Test: cd datastore && ./gradlew bOS
Change-Id: I0b1eb742952b3a1ba451fc30b6cf7f0238a93655
diff --git a/datastore/OWNERS b/datastore/OWNERS
index 2fa7fba..12a6371 100644
--- a/datastore/OWNERS
+++ b/datastore/OWNERS
@@ -1,3 +1,5 @@
 # Bug component: 907884
 [email protected]
[email protected]
\ No newline at end of file
[email protected]
+
+per-file settings.gradle = [email protected], [email protected]