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

tools/releasetools/apex_utils.py

@@ -142,6 +142,7 @@ class ApexApkSigner(object):
       has_signed_content = True
 
     if self.sign_tool:
+      logger.info('Signing payload contents in apex %s with %s', self.apex_path, self.sign_tool)
       cmd = [self.sign_tool, '--avbtool', self.avbtool, payload_key, payload_dir]
       common.RunAndCheckOutput(cmd)
       has_signed_content = True