Fix @Nullability mismatch with PipTransition

Most of the codebase injecting `PipTransition` expects a non-null object. Trying
to inject a null will outright crash. Moreover, nullability mismatch breaks
sysui-studio builds which actually properly typecheck this nullability.

This CL provides legacy transition until non-null pip2.PipTransition is available.

Bug: 293844330
Test: verified the sysui-studio build now builds and runs
Change-Id: I330ad86a70ddfbf0d08bd6d2ed408d6856ca3162

libs/WindowManager/Shell/src/com/android/wm/shell/dagger/pip/Pip1Module.java

@@ -16,7 +16,6 @@
 
 package com.android.wm.shell.dagger.pip;
 
-import android.annotation.Nullable;
 import android.content.Context;
 import android.os.Handler;
 
@@ -229,7 +228,6 @@ public abstract class Pip1Module {
 
     @WMSingleton
     @Provides
-    @Nullable
     static PipTransition providePipTransition(Context context,
             ShellInit shellInit, ShellTaskOrganizer shellTaskOrganizer, Transitions transitions,
             PipAnimationController pipAnimationController, PipBoundsAlgorithm pipBoundsAlgorithm,

libs/WindowManager/Shell/src/com/android/wm/shell/dagger/pip/PipModule.java

@@ -36,10 +36,13 @@ import dagger.Provides;
 public abstract class PipModule {
     @WMSingleton
     @Provides
-    @Nullable
     static PipTransitionController providePipTransitionController(
             com.android.wm.shell.pip.PipTransition legacyPipTransition,
-            com.android.wm.shell.pip2.PipTransition newPipTransition) {
-        return PipUtils.isPip2ExperimentEnabled() ? newPipTransition : legacyPipTransition;
+            @Nullable com.android.wm.shell.pip2.PipTransition newPipTransition) {
+        if (PipUtils.isPip2ExperimentEnabled() && newPipTransition != null) {
+            return newPipTransition;
+        } else {
+            return legacyPipTransition;
+        }
     }
 }