Update cancellation to set the activity to RESULT_CANCELED

We currently set the pick activity to RESULT_OK when the cancel button
is clicked. This doesn't idiomatically follow the same expectation as
the back arrow which sets the result CANCELED. Update all the picker
buttons to cancel instead.

Fix: 417598909
Test: atest com.android.documentsui.PickActivityTest
Flag: com.android.documentsui.flags.use_material3
Change-Id: Iee6a8561d3ab3f3b3565e67c1ae152f0a819ec2b