Apply ALLOW_MISSING_DEPENDENCIES to droiddoc template dir

Some of our minimal branches don't have the necessary droiddoc
templates, so don't cause a FindEmulator warning when the template dir
is missing and ALLOW_MISSING_DEPENDENCIES is set. The warnings will soon
be errors.

When the template directory is missing, ensure that the command doesn't
succeed by adding the directory into the dependency list. Passing a
missing directory doesn't cause an error otherwise, it just wouldn't use
the template.

Test: build-aosp_arm64.ninja is identical before and after this change
Test: Move away build/tools/droiddoc/templates-sdk; m -j libcore-docs
Change-Id: I122f3916b6ab348a5a98dcf2520992eda3655e26
diff --git a/core/droiddoc.mk b/core/droiddoc.mk
index 7e17a13..eb12630 100644
--- a/core/droiddoc.mk
+++ b/core/droiddoc.mk
@@ -130,7 +130,13 @@
 ##
 
 droiddoc_templates := \
-    $(sort $(shell find $(LOCAL_DROIDDOC_CUSTOM_TEMPLATE_DIR) -type f))
+    $(sort $(shell find $(LOCAL_DROIDDOC_CUSTOM_TEMPLATE_DIR) -type f $(if $(ALLOW_MISSING_DEPENDENCIES),2>/dev/null)))
+
+ifdef ALLOW_MISSING_DEPENDENCIES
+  ifndef droiddoc_templates
+    droiddoc_templates := $(LOCAL_DROIDDOC_CUSTOM_TEMPLATE_DIR)
+  endif
+endif
 
 droiddoc := \
 	$(HOST_JDK_TOOLS_JAR) \