[Ada] Rename some observer callbacks in ada-tasks.c

[Ada] Rename some observer callbacks in ada-tasks.c

This patch is mostly cosmetic, avoiding us to use the same callback
names as in ada-lang.c.

gdb/ChangeLog:

        * ada-tasks.c (ada_tasks_new_objfile_observer): Renames
        ada_new_objfile_observer.
        (ada_tasks_normal_stop_observer): Renames ada_normal_stop_observer.
        (_initialize_tasks): Update uses of ada_new_objfile_observer
        and ada_tasks_normal_stop_observer.