Add filegroup_defaults module

Allows specifying shared filegroup attributes in a central place.

Test: filegroup_test.go
Change-Id: I82feac37ae6998313a0004f8af58f3decf7a514e
(cherry picked from https://android-review.googlesource.com/q/commit:7d6dd8bb3324e81b5c4afc80b062baad2fed1770)
Merged-In: I82feac37ae6998313a0004f8af58f3decf7a514e