Support persist.sys.audit_safemode

Don't leave safe more if persist.sys.audit_safemode is set, unless the
current build date is newer than the specified value.

This allows us to keep the device in safe mode across reboots until an
OTA has been applied or user data is wiped.

Bug: 26902605
Change-Id: I781c3059ea8d4fb2f0c923e4488b1932d69678d3