Fix incorrect initial position/scale of opening animations.

1. Initial position of opening animation isn't correctly offset if the
position repository isn't properly offset by the action bar height.
2. Initial scale of opening animation should match the thumbnail instead
of using a fixed value 240.
This CL is part of efforts to fix b:6016973.

Change-Id: I620705357ba7594d43685e47488928f9fc93d9f6