Use "switch_to_thread" more thoroughly on gdbserver

Use "switch_to_thread" more thoroughly on gdbserver

This is a technical debt that I left when I ported "switch_to_thread"
to gdbserver.  It's a simple patch that converts occurences of:

  current_thread = find_thread_ptid (ptid);

to:

  switch_to_thread (ptid);

This patch also converts a simple "if" to a "gdb_assert" on
gdbserver's version of "switch_to_thread":

  gdb_assert (ptid != minus_one_ptid);

This change makes the code more similar to what GDB already does.

Regtested on BuildBot.

gdb/gdbserver/ChangeLog:
2017-09-26  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdbthread.h: Include "common-gdbthread.h".
	* inferiors.c (switch_to_thread): Use "gdb_assert" instead of
	"if" when validating the ptid.
	* remote-utils.c: Include "gdbthread.h".
	(prepare_resume_reply): Use "switch_to_thread".
	* target.c (done_accessing_memory): Likewise.