Add qemu.hw.mainkeys to system property_contexts

Bug: 178143857

Signed-off-by: Weilun Du <wdu@google.com>
Change-Id: Idfc2bffa52016d1e880974bb193025400e90a538
diff --git a/private/system_server.te b/private/system_server.te
index e1919e2..479f85e 100644
--- a/private/system_server.te
+++ b/private/system_server.te
@@ -1318,3 +1318,6 @@
 # Only system server can write the font files.
 neverallow { domain -init -system_server } font_data_file:file no_w_file_perms;
 neverallow { domain -init -system_server } font_data_file:dir no_w_dir_perms;
+
+# Read qemu.hw.mainkeys property
+get_prop(system_server, qemu_hw_prop)