Perception: static const member kInitializer is not initialized properly

[jilinzhou]

Describe the bug if macro definition PERCEPTION_BASE_DISABLE_POOL is undefined, object_pool_test.cc fails to build because of undefined linking reference to kInitializer.

ERROR: (12-01 14:35:25.326) /apollo/modules/perception/radar/lib/tracker/common/BUILD:15:1: Linking of rule '//modules/perception/radar/lib/tracker/common:radar_track_test' failed (Exit 1).
bazel-out/local-dbg/bin/_solib_k8/libmodules_Sperception_Sradar_Slib_Stracker_Scommon_Slibradar_Utrack.so: undefined reference to `apollo::perception::base::ConcurrentObjectPool<apollo::perception::base::Object, 10000ul, apollo::perception::base::ObjectInitializer>::kInitializer'
bazel-out/local-dbg/bin/_solib_k8/libmodules_Sperception_Sbase_Slibobject_Upool_Utypes.so: undefined reference to `apollo::perception::base::ConcurrentObjectPool<apollo::perception::base::AttributePointCloud<apollo::perception::base::Point<float> >, 1000ul, apollo::perception::base::PointCloudInitializer<float> >::kInitializer'
bazel-out/local-dbg/bin/_solib_k8/libmodules_Sperception_Sbase_Slibobject_Upool_Utypes.so: undefined reference to `apollo::perception::base::ConcurrentObjectPool<apollo::perception::base::Frame, 100ul, apollo::perception::base::FrameInitializer>::kInitializer'
bazel-out/local-dbg/bin/_solib_k8/libmodules_Sperception_Sbase_Slibobject_Upool_Utypes.so: undefined reference to `apollo::perception::base::ConcurrentObjectPool<apollo::perception::base::AttributePointCloud<apollo::perception::base::Point<double> >, 1000ul, apollo::perception::base::PointCloudInitializer<double> >::kInitializer'
collect2: error: ld returned 1 exit status

Cause: kInitializer is a static const member varible which need to be initialized properly.