Allow statsd to be given empty config.

Statsd clients may want to set an empty config temporarily, so it's
more convenient to allow them to set an empty config instead of
having to use the removeConfig and then having to remember to call
StatsManager#setDataFetchOperation.

Test: Added unit-tests and check they pass on marlin-eng.
Bug: 74997752
Change-Id: I2e762e5ec01e5a2c9a3469fb330b53fefbd734d6
diff --git a/cmds/statsd/Android.mk b/cmds/statsd/Android.mk
index 5d6a1d1a..ca0611b 100644
--- a/cmds/statsd/Android.mk
+++ b/cmds/statsd/Android.mk
@@ -177,6 +177,7 @@
     tests/LogEvent_test.cpp \
     tests/MetricsManager_test.cpp \
     tests/StatsLogProcessor_test.cpp \
+    tests/StatsService_test.cpp \
     tests/UidMap_test.cpp \
     tests/FieldValue_test.cpp \
     tests/condition/CombinationConditionTracker_test.cpp \