Support custom APEX signing tool

When an APEX specifies its custom signing tool (custom_sign_tool:),
apexkeys.txt contains the info and sign_target_files_apks pass the value
to apex_util.

For now the Virt APEX has its own custom signing tool (sign_virt_apex),
which is added to OTATOOLS.

Bug: 193504286
Test: sign_target_files_apks invokes sign_virt_apex
Change-Id: Iba845723fe3e18f542963324b9c58cd00914c5ba
diff --git a/core/Makefile b/core/Makefile
index 2e72727..0b55c55 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -4323,6 +4323,7 @@
   shflags \
   sign_apex \
   sign_target_files_apks \
+  sign_virt_apex \
   signapk \
   simg2img \
   sload_f2fs \