[autotest] Convert dut_status to the new get_status_task RPC.

This change converts the dut_status command and other clients of the
status_history module to use the new get_status_task() RPC call.
This call offers improved accuracy of diagnosis, and is
significantly faster than the previous method.

This change can't be committed until the previous change enabling
the RPC call is pushed to prod.

BUG=hromium:463632
TEST=Run dut_status using a local instance pointed at the prod database

Change-Id: I0a686b68498938a2cad747ff1f82152d3ea6d48a
Reviewed-on: https://chromium-review.googlesource.com/266096
Reviewed-by: Mungyung Ryu <mkryu@google.com>
Tested-by: Richard Barnette <jrbarnette@chromium.org>
Commit-Queue: Richard Barnette <jrbarnette@chromium.org>
diff --git a/server/frontend.py b/server/frontend.py
index 945aa28..82cfcfe 100644
--- a/server/frontend.py
+++ b/server/frontend.py
@@ -321,6 +321,12 @@
         return [SpecialTask(self, t) for t in tasks]
 
 
+    def get_status_task(self, host_id, end_time):
+        task = self.run('get_status_task',
+                         host_id=host_id, end_time=end_time)
+        return SpecialTask(self, task) if task else None
+
+
     def create_job_by_test(self, tests, kernel=None, use_container=False,
                            kernel_cmdline=None, **dargs):
         """