ANDROID: rust_binder: fix CFI failure in rust_shrink_free_page

Bindgen currently turns C enums into a typedef for `unsigned int`. This
normally works, but means that rustc will generate the wrong CFI tags
for functions that use C enums as parameters or return types.

To work around this, introduce an intermediate C function that
"converts" from C enum to unsigned int.

Bug: 335105888
Change-Id: I5d49e69d24333c1f421a4e579c04ca38019a303b
Signed-off-by: Alice Ryhl <aliceryhl@google.com>