Allow system_server to remove files in /data/misc/wmtrace/*

Bug: 64831661
Test: adb shell cmd window tracing start && adb shell cmd window tracing stop
Test: adb shell su root dmesg | grep 'avc: '
Change-Id: I1578aac9e102246ec722c78a6e9efb5581259d81

private/system_server.te

@@ -642,9 +642,9 @@ userdebug_or_eng(`
   # Allow system server to read dmesg
   allow system_server kernel:system syslog_read;
 
-  # Allow writing window traces in /data/misc/wmtrace.
+  # Allow writing and removing window traces in /data/misc/wmtrace.
   allow system_server wm_trace_data_file:dir rw_dir_perms;
-  allow system_server wm_trace_data_file:file { getattr setattr create w_file_perms };
+  allow system_server wm_trace_data_file:file { getattr setattr create unlink w_file_perms };
 ')
 
 # For AppFuse.