Set documentLaunchMode to alias activity

The alias activity doesn't have the documentLaunchMode information.
So, an new activity instance was created while starting the alias
activity of a documentLaunchMode “intoExisting” activity even with
the same Uri.

This CL updates the documentLaunchMode from target activity to alias
activity.

Bug: 128526493
Test: atest ActivityStarterTests
      1. Launch an intoExisting activity with data uri
      2. Launch an alisa activity with data uri.
      3  Check if launch the existing activity
Change-Id: I256e1db99363bd5950ebe01aeec2d37c7eba4722