Return the header jar of the implementation library in SdkHeaderJars()

As part of the process to remove the compilation action in the top level
java sdk library, this change modifies SdkHeaderJars(...) (i.e. the
"magic") to return the header jar of the implementation library instead
of the header jar of the top level java sdk library when the
implementation jar is returned from the "magic".

This change also removes `SdkImplementationJars()` from the
SdkLibraryDependency interface, as it is not currently used anywhere
else.

Test: m nothing --no-skip-soong-tests
Bug: 332785297
Change-Id: Icc00af4b1485dc2b1b0981a3e56758d0306dea69