Do not append NEW_TASK flag when caller activity is finishing

The caller activity and its task should be able to reuse. There's
no need to migrate the launching activity to another Task.

Bug: 233591548
Bug: 260978212
Test: wm presubmit
Change-Id: I2bd5f01ef9196010f1af1aa7cb9e6a45984e960d
(cherry picked from commit a7269cd7)