Simplify java library sdk member code

Adds the accessor function for retrieving the impl/header jars to the
librarySdkMemberType structure instead of passing it into its
buildSnapshot() method.

That enabled:
* The removal of the [header/impl]LibrarySdkMemberType structs.
* The removal of their implementations of BuildSnapshot.
* Replacing buildSnapshot() with BuildSnapshot()

This will make subsequent refactoring of the SdkMemberType interface
a little simpler.

Bug: 153306490
Test: m nothing
Bug: 150451422
Merged-In: I1f96986bb497cf9d9df9916e40065f66b35a4704
Change-Id: I1f96986bb497cf9d9df9916e40065f66b35a4704