ANDROID: rust_binder: use tgid_nr_ns for getting pid

Because we are backporting commit e0020ba6 ("rust: add
PidNamespace"), it is neccesary to update Rust Binder to use the new
tgid_nr_ns() method for getting the task's pid, as that commit deleted
pid_in_current_ns().

Bug: 429146594
Change-Id: I3acd7e3f1fee2957d5663208614667220e09dfd5
Signed-off-by: Alice Ryhl <aliceryhl@google.com>