Add logs for attempting to seek with only sharedElementTransition

If you set a sharedElementTransition for a FragmentTransaction and do
not also set a transition on the entering or exiting fragment,
DefaultSpecialEffectsController does not run any transitions.

This change is adding logs to indicate to devs exactly why the
transition is not running.

RelNote: "Added logs to indicate why setting a sharedElement without any
other transitions will fail to run."
Test: add exiting tests pass
Fixes: 300657793
(cherry picked from https://android-review.googlesource.com/q/commit:f9678451d5ee7e6627ac57eb753de092caa076d2)
Merged-In: Iec48ef375eb1c498bc13da82d5d8355be58ba0ff
Change-Id: Iec48ef375eb1c498bc13da82d5d8355be58ba0ff

diff --git a/fragment/fragment/src/main/java/androidx/fragment/app/DefaultSpecialEffectsController.kt b/fragment/fragment/src/main/java/androidx/fragment/app/DefaultSpecialEffectsController.kt
index 92eb9d9..9dd1719 100644
--- a/fragment/fragment/src/main/java/androidx/fragment/app/DefaultSpecialEffectsController.kt
+++ b/fragment/fragment/src/main/java/androidx/fragment/app/DefaultSpecialEffectsController.kt

@@ -338,6 +338,13 @@
                 if (sharedElementNameMapping.isEmpty()) {
                     // We couldn't find any valid shared element mappings, so clear out
                     // the shared element transition information entirely
+                    Log.i(FragmentManager.TAG,
+                        "Ignoring shared elements transition $sharedElementTransition between " +
+                            "$firstOut and $lastIn as there are no matching elements " +
+                            "in both the entering and exiting fragment. In order to run a " +
+                            "SharedElementTransition, both fragments involved must have the " +
+                            "element."
+                    )
                     sharedElementTransition = null
                     sharedElementFirstOutViews.clear()
                     sharedElementLastInViews.clear()
@@ -739,6 +746,16 @@
                 }
                 return
             }
+            if (transitioning && sharedElementTransition != null && !isSeekingSupported) {
+                Log.i(FragmentManager.TAG,
+                    "Ignoring shared elements transition $sharedElementTransition between " +
+                        "$firstOut and $lastIn as neither fragment has set a Transition. In " +
+                        "order to run a SharedElementTransition, you must also set either an " +
+                        "enter or exit transition on a fragment involved in the transaction. The " +
+                        "sharedElementTransition will run after the back gesture has been " +
+                        "committed."
+                )
+            }
             if (isSeekingSupported && transitioning) {
                 // We need to set the listener before we create the controller, but we need the
                 // controller to do the desired cancel behavior (animateToStart). So we use this