Block until userdata backup/restore has taken place

Currently, userdata backup/restore is performed on a handler thread.
This means that we cannot guarantee that an application will not begin
running during a backup/restore, as we have no guarantees of when
the backup logic will actually execute.

In the onUnlockUser case we can block until the handler thread has
completed its work by using a CountdownLatch. In the general case we
will keep
snapshotAndRestoreUserData on the handler thread, using Package
Manager's token to decide when to unfreeze

Test: atest RollbackTest
Test: atest StagedRollbackTest
Bug: 124032231
Change-Id: I4c818833e48a549e8a3aa8401539f44c14b89b57

services/core/java/com/android/server/rollback/RollbackManagerServiceImpl.java

@@ -564,6 +564,10 @@ class RollbackManagerServiceImpl extends IRollbackManager.Stub {
     }
 
     void onUnlockUser(int userId) {
+        // In order to ensure that no package begins running while a backup or restore is taking
+        // place, onUnlockUser must remain blocked until all pending backups and restores have
+        // completed.
+        CountDownLatch latch = new CountDownLatch(1);
         getHandler().post(() -> {
             final List<Rollback> rollbacks;
             synchronized (mLock) {
@@ -576,7 +580,14 @@ class RollbackManagerServiceImpl extends IRollbackManager.Stub {
             for (Rollback rollback : changed) {
                 saveRollback(rollback);
             }
+            latch.countDown();
         });
+
+        try {
+            latch.await();
+        } catch (InterruptedException ie) {
+            throw new IllegalStateException("RollbackManagerHandlerThread interrupted");
+        }
     }
 
     private void updateRollbackLifetimeDurationInMillis() {