setExplicitHealthCheckEnabled() needs to set mSyncRequired (6/n)

We need to call mHealthCheckController.syncRequests() whenever
ExplicitHealthCheckEnabled changes. However, syncRequests() fails
to figure this out because it detects changes in
mRequestedHealthCheckPackages only.

Bug: 178675924
Test: atest PackageWatchdogTest
Change-Id: Ic752c7f35c06a0ce770e07d1a186dd9da88b4f1e
diff --git a/services/core/java/com/android/server/PackageWatchdog.java b/services/core/java/com/android/server/PackageWatchdog.java
index 41903fc..ef21a05 100644
--- a/services/core/java/com/android/server/PackageWatchdog.java
+++ b/services/core/java/com/android/server/PackageWatchdog.java
@@ -537,6 +537,7 @@
         synchronized (mLock) {
             mIsHealthCheckEnabled = enabled;
             mHealthCheckController.setEnabled(enabled);
+            mSyncRequired = true;
             // Prune to update internal state whenever health check is enabled/disabled
             syncState("health check state " + (enabled ? "enabled" : "disabled"));
         }