gdb: Use scoped_restore_terminal_state in annotate.c

In a couple of places in annotate.c we are manually backing up and
restoring the terminal ownership, we could instead make use of
scoped_restore_terminal_state.

gdb/ChangeLog:

	* annotate.c (annotate_breakpoints_invalid): Make use of
	scoped_restore_terminal_state.
	(annotate_frames_invalid): Likewise.