OverridableModuleBase shouldn't embed ModuleBase.

Individual module structs normally embed ModuleBase themselves. This
caused duplicate embedding, and so made common properties like notice
inaccessible.

Bug: 135460391
Test: app_test.go, TreeHugger
Change-Id: I5683b194e3de909d480c083a931cc7e871de1d74
Merged-In: I5683b194e3de909d480c083a931cc7e871de1d74