Binder: cleanup "warning: manual implementation of `Option::map`"

[ojeda]

warning: manual implementation of `Option::map`
   --> drivers/android/process.rs:189:12
    |
189 |           Ok(match self.get_existing_node(ptr, cookie)? {
    |  ____________^
190 | |             None => None,
191 | |             Some(node) => Some(self.new_node_ref(node, strong, thread)),
192 | |         })
    | |_________^ help: try this: `self.get_existing_node(ptr, cookie)?.map(|node| self.new_node_ref(node, strong, thread))`
    |
    = note: `#[warn(clippy::manual_map)]` on by default
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_map