Hide 'Optimizing apps' dialog if not M->N upgrade

Recent changes reduced the amount of time spent optimizing apps at
post-OTA boot. Instead of briefly showing the 'Optimizing apps'
dialog, we hide it completely.

The dialog is still shown for OTAs from a pre-N version of Android
since they can take a non-negligible amount of time (still much less
than in M).

Bug: 27350503
Change-Id: I5f6dae0c69799e1fc522f36453d2671b333bef0e

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

@@ -7187,7 +7187,7 @@ public class PackageManagerService extends IPackageManager.Stub {
                         numberOfPackagesToDexopt + ": " + pkg.packageName);
             }
 
-            if (!isFirstBoot()) {
+            if (mIsPreNUpgrade) {
                 try {
                     ActivityManagerNative.getDefault().showBootMessage(
                             mContext.getResources().getString(R.string.android_upgrading_apk,