Add soong variable for PRODUCT_FULL_TREBLE.

Required to preserve sane defaults for old codebases.

Test: inspect soong.variables
Test: ensure PRODUCT_SHIPPING_API_LEVEL is set in config.mk
Bug: 38415912
Change-Id: I0244fb02754663be3106391c9360fd1b2cc00d4a
diff --git a/core/soong_config.mk b/core/soong_config.mk
index 0a2208b..977b35c 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -80,6 +80,7 @@
 	echo '    "EnableCFI": $(if $(filter false,$(ENABLE_CFI)),false,true),'; \
 	echo '    "Device_uses_hwc2": $(if $(filter true,$(TARGET_USES_HWC2)),true,false),'; \
 	echo '    "Override_rs_driver": "$(OVERRIDE_RS_DRIVER)",'; \
+	echo '    "Treble": $(if $(filter true,$(PRODUCT_FULL_TREBLE)),true,false),'; \
 	echo ''; \
 	echo '    "ArtUseReadBarrier": $(if $(filter false,$(PRODUCT_ART_USE_READ_BARRIER)),false,true),'; \
 	echo ''; \