droiddoc: Fix trailing ':' in -classpath, -sourcepath values

full_java_lib_deps has trailing whitespace if $(LOCAL_CLASSPATH)
is empty. Because the calculation of PRIVATE_CLASSPATH was
missing a $(strip ...), this resulted in an (unintentional)
trailing ':' in the values of the -classpath and -sourcepath
arguments to the javadoc tool, which that tool is documented
to interpret as '.' (the working directory).

This CL converts the expression to a call to normalize-path-list,
from definitions.mk, which contains the $(strip ...):

define normalize-path-list
$(subst $(space),:,$(strip $(1)))
endef

After this CL, an empty $(LOCAL_CLASSPATH) no longer gets
misinterpreted as the current working directory.

This issue was minor (it made no difference in practice).

Test: Treehugger
Bug: 62049770

Change-Id: Ia0e3e5657d0fa057fe998515f34bc7b8df5f6f16
diff --git a/core/droiddoc.mk b/core/droiddoc.mk
index 48a930d..4b0ec25 100644
--- a/core/droiddoc.mk
+++ b/core/droiddoc.mk
@@ -86,8 +86,7 @@
 full_java_lib_deps := $(call java-lib-deps,$(LOCAL_JAVA_LIBRARIES)) $(LOCAL_CLASSPATH)
 endif # !LOCAL_IS_HOST_MODULE
 
-$(full_target): PRIVATE_CLASSPATH := $(subst $(space),:,$(full_java_libs))
-
+$(full_target): PRIVATE_CLASSPATH := $(call normalize-path-list,$(full_java_libs))
 
 intermediates.COMMON := $(call local-intermediates-dir,COMMON)