Prevent NPEs when no focusable stack.

Fallback to home stack when no focusable stack
(e.g. launcher force stopped & enter PIP mode with no other app task),
Fix NPEs for logWindowState & getTopRunningActivity from null stack.

Change-Id: I0a0134661dfc20ff844a0bc19ce6833242f4a865
Fixes: 110078217
Test: Manual
Test: atest ActivityStackSupervisorTests ActivityStackTests ActivityStarterTests

services/core/java/com/android/server/am/ActivityMetricsLogger.java

@@ -185,6 +185,10 @@ class ActivityMetricsLogger {
 
         mWindowState = WINDOW_STATE_INVALID;
         ActivityStack stack = mSupervisor.getFocusedStack();
+        if (stack == null) {
+            return;
+        }
+
         if (stack.isActivityTypeAssistant()) {
             mWindowState = WINDOW_STATE_ASSISTANT;
             return;

services/core/java/com/android/server/am/ActivityStackSupervisor.java

@@ -708,6 +708,11 @@ public class ActivityStackSupervisor extends ConfigurationContainer implements D
         if (!focusCandidate.isFocusable()) {
             // The focus candidate isn't focusable. Move focus to the top stack that is focusable.
             focusCandidate = getNextFocusableStackLocked(focusCandidate, false /* ignoreCurrent */);
+            if (focusCandidate == null) {
+                Slog.w(TAG,
+                        "setFocusStackUnchecked: No focusable stack found, focus home as default");
+                focusCandidate = mHomeStack;
+            }
         }
 
         if (focusCandidate != mFocusedStack) {