Refactor ScreenCaptureListenerWrapper to guarantee callback delivery

The previous implementation of ScreenCaptureListenerWrapper used a weak
reference to hold the Java consumer object.

This created a race where the Java consumer could be garbage collected
before the native onScreenCaptureCompleted() callback was invoked. In
that case the screenshot result will be sliently dropped.

This change uses a strong global reference to the Java consumer object,
to ensure the consumer remains alive until the native callback is
invoked. The strong reference will be deleted immediately after the
callback is fired, as the listener is a one-shot mechanism.

Test: presubmit and locally tested screenshot functions.
Test: adding logs and see if the global reference deletion is called
after screenshot finishes.

Bug: 441019220
Flag: EXEMPT BUGFIX

Change-Id: I8e907384b915c30a7b9c1927b1cd517da26561e3