Use depsets for transitive manifests and assets

Instead of rolling assets and manifests up through each static lib,
provide them as a DepSet.  This will help with the next patch, which
needs to pass all the transitive manifests and R.txt files together.

Test: app_test.go
Change-Id: Id8b3aa2bed3771e82ab6bde192c9b43baa38b54c