Fixing a bug introduced due to refactoring var names

SELinux messages weren't properly filtering into the events buffer
because message_len was being used to determine length of event data
instead of str_len

Bug: 65597269
Test: SELinux denials show up in the events buffer
Change-Id: I857e10211f71dd0db33a272241c7051c5d0c59ca
(cherry picked from commit 3a5acdaa)