Register and unregister might be out of order (5/n)

Calling unregisterHealthObserver() right after startObservingHealth()
will fail to unregister observers correctly because startObservingHealth()
do registration asynchronously.

Let's do unregistration asynchronously as well to keep things in order.

Bug: 178675924
Test: atest PackageWatchdogTest
Change-Id: I7dfea957fb88f0346cdeea20e626a728ec1a1325

services/core/java/com/android/server/PackageWatchdog.java

@@ -372,10 +372,12 @@ public class PackageWatchdog {
      * even from a previous boot.
      */
     public void unregisterHealthObserver(PackageHealthObserver observer) {
-        synchronized (mLock) {
-            mAllObservers.remove(observer.getName());
-        }
-        syncState("unregistering observer: " + observer.getName());
+        mLongTaskHandler.post(() -> {
+            synchronized (mLock) {
+                mAllObservers.remove(observer.getName());
+            }
+            syncState("unregistering observer: " + observer.getName());
+        });
     }
 
     /**