Use file_contexts for flattened apexes

Because flattened apexes are installed in /<partition>/apex, associated
file_contexts file should be modified to accommodate path changes and
also merged into target-wide file_contexts.bin file.

Soong now emits modified file_contexts via LOCAL_FILE_CONTEXTS, and
these files are collected into a temporary file_contexts.modules.tmp
file, which is used as an additional input of file_contexts.bin.

Bug: 166518492
Test: m file_contexts.bin
      check the contents of file_contexts.concat.tmp file
Change-Id: I002a2294af9e06baeaf54bc5a9a5380d23b8a3da
diff --git a/core/base_rules.mk b/core/base_rules.mk
index 58be7a2..adf61f1 100644
--- a/core/base_rules.mk
+++ b/core/base_rules.mk
@@ -987,6 +987,9 @@
 ALL_MODULES.$(my_register_name).TEST_CONFIG := $(test_config)
 ALL_MODULES.$(my_register_name).EXTRA_TEST_CONFIGS := $(LOCAL_EXTRA_FULL_TEST_CONFIGS)
 ALL_MODULES.$(my_register_name).TEST_MAINLINE_MODULES := $(LOCAL_TEST_MAINLINE_MODULES)
+ifndef LOCAL_IS_HOST_MODULE
+ALL_MODULES.$(my_register_name).FILE_CONTEXTS := $(LOCAL_FILE_CONTEXTS)
+endif
 test_config :=
 
 INSTALLABLE_FILES.$(LOCAL_INSTALLED_MODULE).MODULE := $(my_register_name)