--sign_tool is an optional flag applicable to virtApex.

For not virtApex, this results in a KeyError.
line 151, in <module>\n', '  File
"/usr/local/google/home/baligh/clients/goog/master/out/host/linux-x86/bin/sign_apex/sign_apex.py",
line 144, in main\n', "KeyError: 'sign_tool'\n"]

BUG: 193504286
Test: TH
Change-Id: Id982e5c57086ada78168163d2293813df121847d