android: Use event identifier instead of userdata pointer (#1826)

ndk-glue currently sets both the `ident` field and user-data pointer to
`0` or `1` for the event pipe and input queue respectively, to tell
these sources apart. While it works to reinterpret this `data` pointer
as integer identifier it shouldn't be abused for that, in particular
when one may wish to provide extra information with an event in the
future; then the `data` field is used as pointer (or abused as abstract
value) for that.

src/platform_impl/android/mod.rs

@@ -30,7 +30,7 @@ enum EventSource {
 
 fn poll(poll: Poll) -> Option<EventSource> {
     match poll {
-        Poll::Event { data, .. } => match data as usize {
+        Poll::Event { ident, .. } => match ident {
             0 => Some(EventSource::Callback),
             1 => Some(EventSource::InputQueue),
             _ => unreachable!(),