rust: Allow rust_ffi_shared in jni_libs

Allow listing rust_ffi_shared modules as a jni_libs dependency
in conjunction with platform_api: true. This allows inclusion by
android_app modules.

Bug: 237304791
Test: android_app module builds with a rust_ffi_shared dependency.
Change-Id: I3a28e1baa522ad8f9c2aa86f1d23b19ce9f967e1