Pack file_contexts into target_files zip.

file_contexts (specified by SELINUX_FC) is needed both when building
and (re)packaging. We used to use the copy in out/ when building, and
looked for the copy in BOOT/RAMDISK/ when packaging from target_files
zip. With system_root_image enabled, the file_contexts needed for
building and packaging might be different from the one on device. So
we explicitly pack the file as META/file_contexts in target_files zip.

Also refactor out the overriding of selinux_fc property into
common.LoadInfoDict().

Change-Id: I94f9ea6671b3792c12c1c21573840743d63da39a
(cherry picked from commit aa7318c3849095aeb3bea00efbf303c0c40a089d)
diff --git a/core/Makefile b/core/Makefile
index e17c4ed..7d039a5 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1454,6 +1454,7 @@
 	$(hide) $(ACP) $(APKCERTS_FILE) $(zip_root)/META/apkcerts.txt
 	$(hide) if test -e $(tool_extensions)/releasetools.py; then $(ACP) $(tool_extensions)/releasetools.py $(zip_root)/META/; fi
 	$(hide) echo "$(PRODUCT_OTA_PUBLIC_KEYS)" > $(zip_root)/META/otakeys.txt
+	$(hide) $(ACP) $(SELINUX_FC) $(zip_root)/META/file_contexts
 	$(hide) echo "recovery_api_version=$(PRIVATE_RECOVERY_API_VERSION)" > $(zip_root)/META/misc_info.txt
 	$(hide) echo "fstab_version=$(PRIVATE_RECOVERY_FSTAB_VERSION)" >> $(zip_root)/META/misc_info.txt
 ifdef BOARD_FLASH_BLOCK_SIZE