Do not override MULTIPLE_TASK flag for singleInstancePerTask

MULTIPLE_TASK flag was removed when documentLaunchMode set to never, but
it shouldn't be removed when launch mode is singleInstancePerTask, or the
behavior is actually duplicated as singleTask.

Bug: 175354896
Test: atest IntentTests
Change-Id: Ib5a94423e4d91f7d93c7fe0fc30e1c91e6616677