metricsd: move timer.h into include/metrics/

Move timer.h into the include/metrics/ folder since some
source files in shill need to include this header file.
Adjust the timer.h include paths in the metricsd accordingly.

BUG: None
Change-Id: Id2f5e079d3a095f40d0f4d4e0d438bd22f1274ff
TEST: "mma" in metricsd folder succeeds.
5 files changed