Add an option to retrieve only packages on system images

Make it easy to observe changes to package states as we do refactoring
on scan/install flows.

Bug: 225756739
Test: adb shell pm list packages --factory-only
Change-Id: Iff8d0b59277d0925473756e07f0252d1d6f8d392

services/core/java/com/android/server/pm/PackageManagerShellCommand.java

@@ -907,6 +907,9 @@ class PackageManagerShellCommand extends ShellCommand {
                         getFlags |= PackageManager.MATCH_APEX;
                         listApexOnly = true;
                         break;
+                    case "--factory-only":
+                        getFlags |= PackageManager.MATCH_FACTORY_ONLY;
+                        break;
                     case "--user":
                         defaultUserId = UserHandle.parseUserArg(getNextArgRequired());
                         break;