pick location of apksigner from PATH if it's not found in the sdk dir
pick location of apksigner from PATH if it's not found in the sdk dir

Summary:
I don't have apksigned available under $ANDROID_SDK/build-tools/23.0.1/apksigner
and as a result `//fbandroid/native/redex:fb-inject-debug` crashes. With this
change I can run the tool with apksigner on PATH, see the test plan.

Reviewed By: justinjhendrick

Differential Revision: D21529509

fbshipit-source-id: 61cdcaf6596199c173f1436b6787104494bbf253

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex