test: Add dbus handler unit tests

Created unit tests for all of the object mapper dbus handlers.

Refactor the handlers to handler.cpp to allow unit testing.
Also updated to use std::set_intersection.

Change-Id: Ib87ab81f949d27f7dab55899d3e8d380e43bda04
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/meson.build b/meson.build
index a7f3a4f..124b37a 100644
--- a/meson.build
+++ b/meson.build
@@ -94,6 +94,7 @@
         'src/main.cpp',
         'src/processing.cpp',
         'src/associations.cpp',
+        'src/handler.cpp',
     ],
     dependencies: [
         boost,