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/tools/releasetools/common.py b/tools/releasetools/common.py
index 5affa32..2ee4b8e 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -2107,7 +2107,7 @@
   devnull = open("/dev/null", "w+b")
   for k in sorted(keylist):
     # We don't need a password for things that aren't really keys.
-    if k in SPECIAL_CERT_STRINGS:
+    if k in SPECIAL_CERT_STRINGS or k is None:
       no_passwords.append(k)
       continue