Add system service for accessing SystemConfig

Add SystemConfigService and associated AIDLs and permissions for
accessing SystemConfig. The service returns values obtained from a
static instance of com.android.server.SystemConfig.

Bug: 143112379
Test: atest SystemConfigTest
Change-Id: I1a863ae9f53db21d698376008e5b1da83309b141

services/java/com/android/server/SystemServer.java

@@ -887,6 +887,11 @@ public final class SystemServer {
     private void startCoreServices(@NonNull TimingsTraceAndSlog t) {
         t.traceBegin("startCoreServices");
 
+        // Service for system config
+        t.traceBegin("StartSystemConfigService");
+        mSystemServiceManager.startService(SystemConfigService.class);
+        t.traceEnd();
+
         t.traceBegin("StartBatteryService");
         // Tracks the battery level.  Requires LightService.
         mSystemServiceManager.startService(BatteryService.class);