performance: fixup missing const reference in zone

Zone::getSensor is passed directly to the Sensor Manager getSensor which
takes the parameter by reference.  Make it use an explicit const
reference at both layers.

Change-Id: I4895ea2935d20b73b88d33972e44b9ac557cd988
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/test/zone_mock.hpp b/test/zone_mock.hpp
index 8dbf24a..79947fb 100644
--- a/test/zone_mock.hpp
+++ b/test/zone_mock.hpp
@@ -16,5 +16,5 @@
     MOCK_CONST_METHOD0(getMaxRPMRequest, float());
     MOCK_CONST_METHOD0(getFailSafeMode, bool());
     MOCK_CONST_METHOD0(getFailSafePercent, float());
-    MOCK_METHOD1(getSensor, Sensor*(std::string));
+    MOCK_METHOD1(getSensor, Sensor*(const std::string&));
 };