Allow update_engine to log to both logd and filesystem

Add a new option "logtofile" which default to false.
1. If both "logtostderr" and "logtofile" present, write to system debug
and log file.
2. If only "logtostderr" presents, write to system debug.
3. Otherwise write to log file only.

This keep the old logging behavior for omaha.

Bug: 65568605
Test: apply an update on pixel2 and check the log.
Change-Id: Id998f626fe5ebefbf6f4dffc19aee1c552343ab6