libmems: Add IioDevice::ReadEvents

This commit adds ReadEvents function in IioDevice to read events in
raw.

BUG=chromium:971175
TEST=build with debug code in mems_setup/main.cc and camera/hal_adapter
     observe events are retrieved

Change-Id: I79cb477d91e0a4b1fa19d9803b51f036abcedc3b
Reviewed-on: https://chromium-review.googlesource.com/1644833
Tested-by: Cheng-Hao Yang <chenghaoyang@chromium.org>
Commit-Ready: Cheng-Hao Yang <chenghaoyang@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Heng-ruey Hsu <henryhsu@google.com>
diff --git a/libmems/iio_context_impl.h b/libmems/iio_context_impl.h
index bcb48e0..a5a5373 100644
--- a/libmems/iio_context_impl.h
+++ b/libmems/iio_context_impl.h
@@ -31,8 +31,8 @@
 
   iio_context* GetCurrentContext() const;
 
-  std::map<std::string, std::unique_ptr<IioDevice>> devices_;
   std::vector<ContextUniquePtr> context_;
+  std::map<std::string, std::unique_ptr<IioDevice>> devices_;
 
   DISALLOW_COPY_AND_ASSIGN(IioContextImpl);
 };