Check intent action in OMS.PackageReceiver for secure coding

NullPointerException occurs when OMS receives an intent with null action.
In constructor of OverlayManagerService, OMS.PackageReceiver is registered
with data scheme "package".
If a malicious app send broadcast intent only with data scheme "package",
NPE occurs because OMS.PackageReceiver does not check
whether intent.getAction() is null or not.
So add a logic to ignore intent with null action for secure coding.

Test: send broadcast without action like below.
 Intent intent = new Intent();
 Uri uri = Uri.parse("package:com.test");
 intent.setData(uri);
 intent.addFlags(0x01000000);
 sendBroadcast(intent);

Change-Id: I654f54a8a685de2ab985b87f53ad07c4e27db09d
Signed-off-by: Youngha Park <yh007.park@samsung.com>

services/core/java/com/android/server/om/OverlayManagerService.java

@@ -321,6 +321,11 @@ public final class OverlayManagerService extends SystemService {
     private final class PackageReceiver extends BroadcastReceiver {
         @Override
         public void onReceive(@NonNull final Context context, @NonNull final Intent intent) {
+            final String action = intent.getAction();
+            if (action == null) {
+                Slog.e(TAG, "Cannot handle package broadcast with null action");
+                return;
+            }
             final Uri data = intent.getData();
             if (data == null) {
                 Slog.e(TAG, "Cannot handle package broadcast with null data");
@@ -338,7 +343,7 @@ public final class OverlayManagerService extends SystemService {
                 userIds = new int[] { UserHandle.getUserId(extraUid) };
             }
 
-            switch (intent.getAction()) {
+            switch (action) {
                 case ACTION_PACKAGE_ADDED:
                     if (replacing) {
                         onPackageUpgraded(packageName, userIds);