Remove weak reference to prevent loss of callback

Callback was passed as a method reference and stored in
a weak reference. This bug was hidden until ag/15819081 (no longer
block for render thread) was merged. With this change, the callback
now has a higher chance of being collected early and breaking capture.

Bug: 201158402
Fix: 201158402
Test: atest ScrollCaptureConnectionTest
Change-Id: I294a7e3b4e57f60117ea14492182a690f46f02bb