[DatePicker] Fix edge-to-edge mode being applied multiple times

When the date picker goes back from background to foreground, the onStart() method will be called and make enableEdgeToEdge() be called again. This will make it incorrectly use the header height and top padding already adjusted with system inset as the original height and padding, and thus add the inset multiple times.

Creates a flag to only enable edge-to-edge once to fix the issue.

Resolves #2628

PiperOrigin-RevId: 440110562
(cherry picked from commit c6a654c)

lib/java/com/google/android/material/datepicker/MaterialDatePicker.java

@@ -141,6 +141,8 @@ public String getHeaderText() {
   @Nullable private MaterialShapeDrawable background;
   private Button confirmButton;
 
+  private boolean edgeToEdgeEnabled;
+
   @NonNull
   static <S> MaterialDatePicker<S> newInstance(@NonNull Builder<S> options) {
     MaterialDatePicker<S> materialDatePickerDialogFragment = new MaterialDatePicker<>();
@@ -307,7 +309,7 @@ public void onStart() {
     if (fullscreen) {
       window.setLayout(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT);
       window.setBackgroundDrawable(background);
-      enableEdgeToEdge(window);
+      enableEdgeToEdgeIfNeeded(window);
     } else {
       window.setLayout(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
       int inset =
@@ -356,7 +358,11 @@ public final S getSelection() {
     return getDateSelector().getSelection();
   }
 
-  private void enableEdgeToEdge(Window window) {
+  private void enableEdgeToEdgeIfNeeded(Window window) {
+    if (edgeToEdgeEnabled) {
+      // Avoid enabling edge-to-edge multiple times.
+      return;
+    }
     final View headerLayout = requireView().findViewById(R.id.fullscreen_header);
     EdgeToEdgeUtils.applyEdgeToEdge(
         window, true, ViewUtils.getBackgroundColor(headerLayout), null);
@@ -380,6 +386,7 @@ public WindowInsetsCompat onApplyWindowInsets(View v, WindowInsetsCompat insets)
             return insets;
           }
         });
+    edgeToEdgeEnabled = true;
   }
 
   private void updateHeader() {