Disable WITHOUT_CHECK_API for SDK products

SDK products, which are required to build the SDK, require checkapi
to run in order to generate the exportable artifacts, which are copied
to the dist directory. Thus, this change adds a conditional to emit an
error when WITHOUT_CHECK_API environment variable is set when the target
product is a SDK variant.

Test: lunch sdk-trunk_staging-eng && m WITHOUT_CHECK_API=true and observe the expected error being emitted
Bug: 329374072
Change-Id: I0dc628742c49845792b03f9af1ea60cd9eb4b8c3

target/product/sdk.mk

@@ -29,4 +29,8 @@ PRODUCT_NAME := sdk
 PRODUCT_BRAND := Android
 PRODUCT_DEVICE := mainline_x86
 
-PRODUCT_BUILD_FROM_SOURCE_STUB := true
+PRODUCT_BUILD_FROM_SOURCE_STUB := true
\ No newline at end of file
+
+ifeq ($(WITHOUT_CHECK_API),true)
+  $(error WITHOUT_CHECK_API cannot be set to true for SDK product builds)
+endif