Restrict access to mGroupFlags.

We can safely restrict the access because there are no flags that
could be publicly accessed. Please see the overview below.

Private flags:
FLAG_CLIP_TO_PADDING
FLAG_RUN_ANIMATION
FLAG_PADDING_NOT_NULL
FLAG_NOTIFY_ANIMATION_LISTENER
FLAG_ANIMATION_CACHE
FLAG_ALWAYS_DRAWN_WITH_CACHE
FLAG_CHILDREN_DRAWN_WITH_CACHE
FLAG_NOTIFY_CHILDREN_ON_DRAWABLE_STATE_CHANGE
FLAG_MASK_FOCUSABILITY
FLAG_ADD_STATES_FROM_CHILDREN
FLAG_NOTIFY_CHILDREN_ON_DRAWABLE_STATE_CHANGE
FLAG_MASK_FOCUSABILITY
FLAG_START_ACTION_MODE_FOR_CHILD_IS_TYPED
FLAG_START_ACTION_MODE_FOR_CHILD_IS_NOT_TYPED
FLAG_SHOW_CONTEXT_MENU_WITH_COORDS
DESCENDANT_FOCUSABILITY_FLAGS

Package private flags:
FLAG_CLIP_CHILDREN
FLAG_INVALIDATE_REQUIRED
FLAG_ANIMATION_DONE
FLAG_OPTIMIZE_INVALIDATE
FLAG_CLEAR_TRANSFORMATION
FLAG_IS_TRANSITION_GROUP
FLAG_IS_TRANSITION_GROUP_SET
FLAG_TOUCHSCREEN_BLOCKS_FOCUS

Public but not related to mGroupFlags
FOCUS_BEFORE_DESCENDANTS
FOCUS_AFTER_DESCENDANTS
FOCUS_BLOCK_DESCENDANTS

Protected that are going to be restricted
FLAG_SUPPORT_STATIC_TRANSFORMATIONS - ag/6278964
FLAG_USE_CHILD_DRAWING_ORDER - ag/6278967
FLAG_DISALLOW_INTERCEPT - ag/6278968

Bug: b/123769411
Test: N/A
Change-Id: I7137c09f1274b5e65b45163e88ed914b1288a0eb