rust: Add prefer_rlib property for static libstd.

Adds the prefer_rlib property to allow linking libstd statically for
device rust binaries. This also changes the default behavior of rustlibs
to also prefer rlib linkage. This is because dylibs do not provide
rlib-libstd variants and always link in libstd dynamically. Thus a
binary requesting libstd rlib linkage should not attempt to link against
dylibs that link libstd dynamically.

Bug: 168729404
Test: New Soong test passes.
Change-Id: Idf8dfbbce8fd936f55a3fb323b17a1a7f0ee954e