Fix FAB position not persisting and resetting on control panel visibility changes

[khanhtrancc]

Problem

The Menu/Back floating action button (FAB) has two positioning bugs:

• Dragging it to a custom spot doesn't persist — the position is lost on
  app restart / view recreation, always reverting to the XML-defined
  default (bottom-end, above the control panel).
• The position also resets mid-session whenever the media control panel
  (control_panel) shows or hides during playback. The FAB is
  constrained via app:layout_constraintBottom_toTopOf="@id/control_panel",
  so toggling that view's visibility makes ConstraintLayout re-solve all
  constraints and snap the FAB back to its anchor position, discarding
  the drag offset (which was only ever applied via setX/setY, never
  reflected in layout params).

Fix

Save the dragged position as a fraction (0–1) of the available drag
range, rather than raw pixels, and restore it on every layout pass:

• Persist posX/posY fractions via a small dedicated
  SharedPreferenceStore, using the same PreferenceStore/Pref
  abstraction already used elsewhere in the codebase (e.g.
  MainActivityPrefs's percentage-based prefs).
• Factor the drag-bounds calculation (previously inline in
  ACTION_MOVE) into a shared getDragBounds() helper, reused for
  clamping, saving, and restoring.
• Save the fraction on ACTION_UP after a real drag.
• Restore it from a new onLayout() override, which fires both on view
  recreation and on control-panel visibility changes — and is skipped
  while a drag is in progress so it can't fight the user's finger.

If the button was never dragged, the fraction stays unset and behavior
is unchanged (default constraint-defined position).

Scope

Change is fully contained to
depends/utils/src/main/java/me/aap/utils/ui/view/FloatingButton.java.