Adding Metrics/protos in the ConnectivityService

Copy following network slice metric's protos from stats/atoms.proto
into ConnectivityService module.
 1. NetworkSliceRequestCountSample
 2. NetworkSliceSessionEnded
 3. NetworkSliceDailyDataUsageReported

Bug: 255564255
Test: statsd_testdrive
Change-Id: I2dda17e05f4a5e7bdc25902b88b1801a954d13e3