Expand local file path to allow users to fix ndk-build

local-source-file-path does not expand to correct file paths in
some circumstances so some users override it.  Therefore
flatbuffers_header_build_rules has been modified to generate rules
that expand LOCAL_SRC_FILES values with flatbuffers_header_build_rules.

Also, this overrides local-source-file-path to allow nest projects
to build when NDK_OUT is set.

Tested:
Verified a dependent project continues to build.

Bug: 25673744
Change-Id: Ic90186fe96d6e4533f9f3b7ca9ef78084de08a7e

android/jni/include.mk

@@ -192,7 +192,7 @@ $(foreach schema,$(1),\
   $(call flatbuffers_header_build_rule,\
     $(schema),$(strip $(2)),$(strip $(3)),$(strip $(4))))\
 $(foreach src,$(strip $(5)),\
-  $(eval $(PORTABLE_LOCAL_PATH)$$(src): \
+  $(eval $(call local-source-file-path,$(src)): \
     $(foreach schema,$(strip $(1)),\
       $(call flatbuffers_fbs_to_h,$(strip $(2)),$(strip $(3)),$(schema)))))\
 $(if $(6),\
@@ -201,11 +201,18 @@ $(if $(6),\
       $(call flatbuffers_fbs_to_h,$(strip $(2)),$(strip $(3)),$(schema)))),)\
 $(if $(7),\
   $(foreach src,$(strip $(5)),\
-      $(eval $(PORTABLE_LOCAL_PATH)$$(src): $(strip $(7)))),)\
+      $(eval $(call local-source-file-path,$(src)): $(strip $(7)))),)\
 $(if $(7),\
   $(foreach dependency,$(strip $(7)),\
       $(eval $(6): $(dependency))),)
 endef
 
+# TODO: Remove when the LOCAL_PATH expansion bug in the NDK is fixed.
+# Override the default behavior of local-source-file-path to workaround
+# a bug which prevents the build of deeply nested projects when NDK_OUT is
+# set.
+local-source-file-path = \
+  $(if $(call host-path-is-absolute,$1),$1,$(realpath $(LOCAL_PATH)/$1))
+
 endif  # FLATBUFFERS_INCLUDE_MK_