Fix a regression in BNC search previous activity inside task.

Regression from 0643ad0a, where there should search previous activity
from task, instead of only search from a task fragment.

Bug: 311336844
Test: launch a single activity A, then launch two adjacet activities
B & C in two task fragment. Verify there is no default back predict
animation when back gesture start on neither B or C.

Change-Id: Id7823194a1f06b02b54184bf632bb51c374e179f