metricsd: move timer_mock.h and metrics_library_mock.h into include/metrics/

Move metrics_library_mock.h and timer_mock.h into the include/metrics/
folder since some source files in shill need to include these header files.

Adjust the include paths in metricsd source files accordingly, and add
the -frtti flag to the libmetrics target so that classes including
these mock headers can determine the type information of the underlying
class being mocked at run-time.

BUG: 22956441
Change-Id: I19820c0d3097c9c4c2b93730d1988ccec563db23
TEST: "mma" in metricsd folder succeeds.