Null and thread safety fixes

Most of these are related to error handling and cancellation. When
shutting down or failing, cancellation signals, binder and view
references are cleared. Callbacks that have been queued must check
for null before invoking. These calls may come from either the
ScrollCaptureCallback (app side) or ScrollCaptureConnection
(SystemUI side).

Bug: 370630810
Bug: 325469181
Bug: 365207886
Bug: 341544887
Bug: 278379299
Test: ScrollCaptureConnectionTest ScrollCaptureControllerTest
Test: ScrollCaptureClientTest
Flag: NONE - null safety fixes, no change to normal behavior
Change-Id: I2ff8390e96b66dd5cbc35e57c2c6a3c05065fa5e