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