export_proguard_spec for libs deps

Add a property to export proguard flags files for libs dependencies.
Currently only proguard flags files from static deps are propagated up
to reverse dependencies, but it is necessary sometimes to have flags
from libs dependencies also be propagated.

Bug: 289087274
Test: go test ./java
Change-Id: Ic0aa22b086792bf322041aa5780db6c4f4eb2770