Link device binaries dynamically by default.

Device binaries currently are linked statically by default. Instead we
should be linking these dynamic by default. To avoid conflicts when
manually specifying rlib dependencies on modules, we always link libstd
dynamically for all device modules except static libraries.

This removes the "prefer_dynamic" property entirely to avoid confusion.

Bug: 165161105
Test: m profcollectd is built dynamically.
Test: cd external/rust/; mma
Test: cd external/crosvm/; mma
Change-Id: I25ac897040acbcc2d97c791a33e8e01610632272