Treat two TaskFragment leashes as one for TASK_FRAGMENT_OPEN/CLOSE

We use the default activity open animation for TaskFragment open.
However, when two split TaskFragments are opening together, it would
look like their separate. We can't promote it to the Task level because
the Task contains both opening and closing TaskFragments.

The workaround here is to offset the left/right TaskFragments to make
them look like one window.

Bug: 201623109
Bug: 196173550
Test: manually open two TaskFragment in split with demo app
Change-Id: Ic391262dcbd7f35d4e5a88588d39c74b4c5d5aae