apex.custom_sign_tool property

A new property indicates a CLI tool to sign the APEX contents. The value
is stored in apexkeys.txt so that releasetool (sign_target_files_apks)
can use it to invoke the tool to re-sign the apex contents.

Bug: 193504286
Test: m out/soong/apexkeys.txt
      com.android.virt.apex line has sign_tool value
Change-Id: Ifd472049b75b5b87c1ed320f5e1190ff65ed54f2