Add userspace_panic_device and userpanic_use

Define type userspace_panic_device and macro userpanic_use for init,
llkd, and system_server to access /dev/userspace_panic - a kernel file
node for userspace processes to request kernel panic.

Bug: 188777408
Change-Id: I1e9d115d85f664aa84bdd6bb4b95bdb48e3aab9a
diff --git a/private/system_server.te b/private/system_server.te
index 5650a7a..13d620d 100644
--- a/private/system_server.te
+++ b/private/system_server.te
@@ -1156,6 +1156,9 @@
 # Allow system server to read profcollectd reports for upload.
 userdebug_or_eng(`r_dir_file(system_server, profcollectd_data_file)')
 
+# Allow use userpanic to request panic.
+allow system_server userspace_panic_device:chr_file w_file_perms;
+
 ###
 ### Neverallow rules
 ###