Change checkpointing retry count to 2

Changing the checkpointing retry count to 2 resolves
issues of apex sessions being reverted if a hard reboot takes
place during a userspace reboot. Previously, a revert would
occur based on what stage of the boot sequence a hard reboot
took place in, now the revert will not occur at all.

Bug: 148270009
Test: atest ApexRollbackTests
Change-Id: Idce2d60f0a25f01a7d081420c937e5c9cec12e5d

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

@@ -1246,7 +1246,7 @@ public class StagingManager {
             try {
                 IStorageManager storageManager = PackageHelper.getStorageManager();
                 if (storageManager.supportsCheckpoint()) {
-                    storageManager.startCheckpoint(1);
+                    storageManager.startCheckpoint(2);
                 }
             } catch (Exception e) {
                 // Failed to get hold of StorageManager