[rust] Add android_dylib cfg flag

Adds an android_dylib cfg when building a Rust crate as a dylib. This
cfg lets us make the hack to add a dependency on std for the
panic_handler and eh_personality conditional on actually building a
dylib.

Test: m nothing
Bug: 193087363
Change-Id: Idad5b98245264f916afa327812a435b368f99321