Add quote to the signing arguments when passing to apexer

We should add quote when passing the signing arguments to apexer. So
the final argument whould look like
apexer ... --signing_args "--signing_helper_with_files=%path"

And the argument parser in avbtool will eventually parse the
"--signing_helper_with_files" correctly.

Bug: 148627666
Test: unittest pass, run a smoke sign_target_files_apk with signing arguments
Change-Id: Ie203d26a508cb98ca94e6c364a2640b681ad79f2