Command-line flag to disable signing with APK Signature Scheme v2.

This change makes signapk not reject the --disable-v2 command-line
flag which may be used by build scripts in some branches. The flag
is currently ignored.

This change is landed separately from the actual support for APK
Signature Scheme v2 because of unbundled branches which use prebuilt
versions of signapk.

Bug: 25794543
Change-Id: I900966244b8b6296b1f443bf98830cc7f7cc81a8