sign_apex --sign_tool

A new argument is a custom signing tool for APEX contents. When
specified, apex_util invokes the tool with payload's key and payload
directory.

For now, the Virt APEX has its own custom signing tool (sign_virt_apex)
to re-sign filesystem images in it.

Bug: 193504286
Test: atest releasetools_test
Test: m sign_apex sign_virt_apex
Test: sign_apex --sign_tool sign_virt_apex --payload_key ..
  --container_key .. resigned.apex
      adb install resigned.apex
      reboot & vm run-app
Change-Id: Ic4d369c2ba42a8295044a0a75e054dc8def93208