cm: Actually have checkapi-cm match against prior release.

Change-Id: I10b88f962d9410687640a40ce7f564e0bd04c2c6
TICKET: CYNGNOS-2303
diff --git a/build/tasks/apicheck.mk b/build/tasks/apicheck.mk
index 3be3cae..4bb60c2 100644
--- a/build/tasks/apicheck.mk
+++ b/build/tasks/apicheck.mk
@@ -25,8 +25,8 @@
 # Run the checkapi rules by default.
 droidcore: checkapi-cm
 
-# Validate against current platform sdk version api text within prebuilts
-cm_last_released_sdk_version := $(CM_PLATFORM_SDK_VERSION)
+# Validate against previous release platform sdk version api text within prebuilts
+cm_last_released_sdk_version := $(shell echo ${CM_PLATFORM_SDK_VERSION}-1 | bc)
 
 .PHONY: check-cm-public-api
 checkapi-cm : check-cm-public-api