Workaround for verifying large APKs.

When an APK is verifier during installation, the recently added
APK Signature Scheme v2 code uncondionally memory-maps the whole file.
This fails for very large APKs, even those which are not signed with
APK Signature Scheme, thus preventing installation of such APKs.

This temporary workaround pretends that the APK is not signed with
APK Signature Scheme v2 if the APK cannot be memory-mapped because
there's insufficient memory.

This workaround will be removed soon, once APK Signature Scheme v2
APK verification logic can handle very large APKs.

Bug: 27613575
Change-Id: I27bad534855fe4bf3e09b1087398ffdd7f98f482