[ActivityManager] Fix NullPointerException in startNextMatchingActivity.

Symptom:
NullPointerException crash while call startNextMatchingActivity.

Root Cause:
Activity starts with FLAG_DEBUG_LOG_RESOLUTION for debug dump.
If the current Activity is the last object of resolves list,
  we can not get the next Activity and aInfo is still null.

Solution:
Check null in debug log dump.

Change-Id: I4c5d14d9acd8d47768b36c95354763841579e050