Group ActivityEmbedding split by Task id

When the app is the host of multiple Tasks, there can be split pairs in
each Task.

Before, we treated all split as in one Task, which can sometimes cause
issue. Now, we group them by Task id, so that they won't affect each
other.

Bug: 207720388
Test: test with Settings deep link
Change-Id: Iaa9865461d984df10c6db237a87ee0903b70e7da