Fix NPE if draging again while animating

The animator cancel calling might cause surfaces released by our
implementation. Moving the cancel call before we make sure surface
is not null.

Fix: 230686609
Test: Open animation scale setting and try drop then drag again
      divider quickly
Test: pass exising tests
Change-Id: I2e94f3ece3f211ceb15f380e4ee9322b8c27f8ac