Bug fixes for APEX PDP metrics

Bug fixes for APEX PDP metrics

1. APEX engine metrics were being initialized to 0 in the
constructor which caused them to reset to 0. Hence, initialization
is removed, and updates are done to metrics only upon:
engineStart/Stop, executionEnter/Exit stages.
2. APEX engine Uptime was only updated when engine stopped,
hence always 0. Also, uptime is redundant since it can be derived
from lastStart timestamp and hence is removed as a metric.
3. Fixed a corner case bug which occurs when all
policies are undeployed the counters are not correct.
4. Fix Unit tests

Issue-ID: POLICY-4044
Signed-off-by: Rashmi Pujar <rashmi.pujar1@bell.ca>
Change-Id: I4d875ff288cf8c242a1851eb105e56dc69b97883