Change the file location for update_engine log

We used to write update_engine logs to /data/misc/update_engine, which
has 'root' permission and is inaccessible to other users. This CL
changes the log file location to a new directory under /data/misc/
created with 'system' permission.

Bug: 65568605
Test: mma
Change-Id: Ife5ef318692a8c74636524753c0fd696f149bab0