perf(android): hand the captured frame over instead of copying it again (#16835)

#16827 made FrameRaw copy each frame while the Image is still valid, and
then copy it a second time into the capturer in take(). Swap the owned
buffer into dst instead: both callers own dst and only read it until
their next call, so the two buffers alternate and take() holds the lock
for constant time. A taken frame is copied once, as before #16827.

Release the buffer whenever capture is enabled or disabled. FrameRaw is
global, so it would otherwise keep a full frame after capture stops.

Note why the would_block_if_equal check never fires, so that nobody
revives it by accident.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

libs/scrap/src/android/ffi.rs

@@ -59,6 +59,7 @@ impl FrameRaw {
     fn set_enable(&mut self, value: bool) {
         self.enable = value;
         self.ready = false;
+        self.data = Vec::new();
     }
 
     fn update(&mut self, data: *mut u8, len: usize) {
@@ -81,12 +82,13 @@ impl FrameRaw {
             return None;
         }
         self.ready = false;
+        // Never true: `last` starts empty and only `would_block_if_equal` fills it. Left off on
+        // purpose: Android sends a frame only when the screen recomposes, so repeats are rare.
         if last.len() == self.data.len() && crate::would_block_if_equal(last, &self.data).is_err()
         {
             return None;
         }
-        dst.clear();
-        dst.extend_from_slice(&self.data);
+        std::mem::swap(dst, &mut self.data);
         Some(())
     }
 }