Enable ADB shell access to ro.serialno

6e4508e6 inadvertently removed access
to ro.serialno and ro.boot.serialno from ADB shell. This is needed for
CTS. This commit thus reinstates the access.

Test: adb shell getprop ro.serialno
Bug: 33700679
Change-Id: I62de44b1631c03fcd64ceabaf33bbaeb869c2851

public/domain.te

@@ -383,6 +383,7 @@ neverallow {
   -init
   -mediadrmserver
   -recovery
+  -shell
   -system_server
 } serialno_prop:file r_file_perms;
 

public/shell.te

@@ -72,6 +72,9 @@ userdebug_or_eng(`
   set_prop(shell, persist_debug_prop)
 ')
 
+# Read device's serial number from system properties
+get_prop(shell, serialno_prop)
+
 # allow shell access to services
 allow shell servicemanager:service_manager list;
 # don't allow shell to access GateKeeper service