Hides the Edit Shortcut top preference when empty.

This fixes an issue where this page was presenting an inaccurate
preference list count to TalkBack because it was including this empty
preference.

(Uses the same flag as other fixes for this same bug, even though the
flag name doesn't make sense here.)

Test: atest EditShortcutsPreferenceFragmentTest
Test: Confirm preference invisible when editing a single feature,
      opened from the Settings app
Test: Confirm preference invisible when editing a single feature,
      opened from the FAB Drag-to-Edit
Test: Confirm preference still visible when editing multiple features,
      opened from the FAB Drag-to-Edit
Test: Confirm TalkBack announces correct item counts in both cases,
      including when editing Magnification which has an "advanced"
      preference that gets replaced with the triple-tap shortcut.
Bug: 318607873
Flag: com.android.settings.accessibility.toggle_feature_fragment_collection_info
Change-Id: I2e1eb7f66022c1a47d1f96a914294fd8da25549c