Only install snapshotctl on debug builds

snapshotctl is a debug tool, which is intended for engineers to inspect
snapshot partition state when OTA fails. It is not meant for production
use, since it requires root, it can't be used in prod devices' adb shell
either.

Test: build, make sure build has snapshotctl
Change-Id: I350b9522f6c18606c3f3c15a8c231a3c41bee18f
diff --git a/target/product/base_system.mk b/target/product/base_system.mk
index 563511f..878a1a2 100644
--- a/target/product/base_system.mk
+++ b/target/product/base_system.mk
@@ -274,7 +274,6 @@
     Shell \
     shell_and_utilities_system \
     sm \
-    snapshotctl \
     snapuserd \
     storaged \
     surfaceflinger \
@@ -491,6 +490,7 @@
     record_binder \
     servicedispatcher \
     showmap \
+    snapshotctl \
     sqlite3 \
     ss \
     start_with_lockagent \