New activity launch mode - singleInstancePerTask

Adding a new activity launch mode `singleInstancePerTask` which
is similar to singleTask that the activity is single instance and
can only be created in task root, but still allowed to be created
in multiple documents or tasks.

Also adding meta-data support of apps that targeting on previous SDK.

Bug: 175354896
Test: IntentTests

Change-Id: I4ba3b2ba6d619677f03e67d3b7da1b5d2a79907f