Add subcontexts field to JSON class loader context representation.

For now the subcontexts are always empty. A follow-up change will
actually populate that field from dependencies' dexpreopt.config files.

Bug: 132357300
Test: lunch aosp_cf_x86_phone-userdebug && m
Change-Id: I6cf0e8363dff4b406cb62ba3d76d566dbfec57e7
diff --git a/core/dex_preopt_odex_install.mk b/core/dex_preopt_odex_install.mk
index c31d4e8..cdd96f6 100644
--- a/core/dex_preopt_odex_install.mk
+++ b/core/dex_preopt_odex_install.mk
@@ -220,8 +220,9 @@
     $(foreach lib, $(2),\
       $(call add_json_map, $(lib)) \
       $(eval file := $(filter %/$(lib).jar, $(call module-installed-files,$(lib)))) \
-      $(call add_json_str, Host,       $(call intermediates-dir-for,JAVA_LIBRARIES,$(lib),,COMMON)/javalib.jar) \
-      $(call add_json_str, Device,     $(call install-path-to-on-device-path,$(file))) \
+      $(call add_json_str, Host,        $(call intermediates-dir-for,JAVA_LIBRARIES,$(lib),,COMMON)/javalib.jar) \
+      $(call add_json_str, Device,      $(call install-path-to-on-device-path,$(file))) \
+      $(call add_json_map, Subcontexts, ${$}) $(call end_json_map) \
       $(call end_json_map)) \
     $(call end_json_map)