Flatten namespace for platform/ code: opescreen::platform → openscreen

Removes the nested "platform" namespace in the platform code. Fixes all
typenames (fixing/removing using namespace statements, removing
unnecessary namespace qualifiers, etc.).

Also, removed the "using UdpSocketUniquePtr = std::unique_ptr<>" since
the alias is no longer necessary (the smart pointer no longer does
anything special at delete time).

Bug: openscreen:90
Change-Id: I0e8589282b4e42530c8c1c95115078d83e3e5f16
Reviewed-on: https://chromium-review.googlesource.com/c/openscreen/+/1948540
Reviewed-by: Jordan Bayles <jophba@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Commit-Queue: Yuri Wiitala <miu@chromium.org>
diff --git a/platform/impl/task_runner.cc b/platform/impl/task_runner.cc
index 8ff92b3..dae79c4 100644
--- a/platform/impl/task_runner.cc
+++ b/platform/impl/task_runner.cc
@@ -9,9 +9,8 @@
 #include "util/logging.h"
 
 namespace openscreen {
-namespace platform {
 
-TaskRunnerImpl::TaskRunnerImpl(platform::ClockNowFunctionPtr now_function,
+TaskRunnerImpl::TaskRunnerImpl(ClockNowFunctionPtr now_function,
                                TaskWaiter* event_waiter,
                                Clock::duration waiter_timeout)
     : now_function_(now_function),
@@ -143,5 +142,4 @@
   return false;
 }
 
-}  // namespace platform
 }  // namespace openscreen