Initialize content resolver in ctor

* In ag/16337218 we moved the initialization of the
context into the ctor instead of init(), but not the
content resolver.
* When we add a callback for OverviewProxyService in
the ctor, it might call the onConnectionChanged()
before NavBarHelper#init() is called, which would lead
to use of a null content resolver in
updateAssistantAvailability()

Bug: 208261632
Change-Id: I306cdf27c7870ad72e80ef401367bb0d6dc32f68

packages/SystemUI/src/com/android/systemui/navigationbar/NavBarHelper.java

@@ -109,6 +109,7 @@ public final class NavBarHelper implements
             UserTracker userTracker,
             DumpManager dumpManager) {
         mContext = context;
+        mContentResolver = mContext.getContentResolver();
         mAccessibilityManager = accessibilityManager;
         mAssistManagerLazy = assistManagerLazy;
         mStatusBarOptionalLazy = statusBarOptionalLazy;
@@ -124,7 +125,6 @@ public final class NavBarHelper implements
     }
 
     public void init() {
-        mContentResolver = mContext.getContentResolver();
         mContentResolver.registerContentObserver(
                 Settings.Secure.getUriFor(Settings.Secure.ASSISTANT),
                 false /* notifyForDescendants */, mAssistContentObserver, UserHandle.USER_ALL);