rust: Fix Properties inheritance for prebuilts

Previously, we had just called AddProperties inside the factory on
libraryDecorator's properties directly. This inadvertantly missed their
MutatedProperties. Inheriting properties from libraryDecorator rather
than baseCompiler will avoid that type of error in the future.

Test: cd external/rust; mma; m crosvm.experimental
Change-Id: Ic08c410e69a8e272d63f9ffbdbe16ba9c3cc3de1