[CheckBox] Fix a11y announcement for indeterminate state.

PiperOrigin-RevId: 467726106
(cherry picked from commit 6f474b3)

Author: leticiarossi

lib/java/com/google/android/material/checkbox/res/values/strings.xml

@@ -26,7 +26,7 @@
     Not checked
   </string>
   <string name="mtrl_checkbox_state_description_indeterminate" description="Description for when checkbox is indeterminate. [CHAR LIMIT=32]">
-    Indeterminate
+    Partially checked
   </string>
 
   <!-- Drawables strings. -->