move dropbox_flags_lib to framework flag list

This change makes dropbox_flags_lib visible to metalava so
metalava won't hide the guarded api.

Since the guarded api are in the framework, this change move this flag
to framework flag list.

Bug: 333995906
Test: lunch sdk-next-eng && m sdk dist and presubmit
Change-Id: I4ae6c4023816d2d4b87a990d949f6094ab6fd7d3