Use top activity for fixed rotation from snapshot starting window

Assume T=trampoline activity, M=main activity.
When launching T into an existing task which contains the existing M:
The task will have
 Top    = T (no window)  = 'activity'
 Bottom = M (has window) = 'topFullscreenActivity'
Because the real top launching activity is T, we should use it
for checking whether to apply fix rotation. Otherwise fixed rotation
will be skipped because M is not the current top activity, which
will cause the snapshot starting window to be half cropped.

Also make sure the trampoline activity has the same rotation as
the snapshot. Because the snapshot starting window will be shown
on the trampoline first and then transfer to the existing activity.

This relates to commit a95bd4a8 that enables snapshot starting window
to take effect across trampoline activity.

Bug: 236200669
Test: Assume A,B are landscape singleTask activities with the same
      task affinity. A will be finished (clear-task-stack) after
      starting B. Launch the app and return to home and launch it
      again. The starting window should not be cropped.

Change-Id: I11cea671c0e6b9a58ba28f28ce0f735eb576c55d