Require signature scheme V2+ for target SDK R+

With the recent sha-1 news, and since minSdk less than 18 for RSA and
minSdk less than 21 for DSA require sha-1, APK signature verification
will now require a minimum of a V2 signature for apps targeting R+.

Bug: 148313868
Test: atest PkgInstallSignatureVerificationTest
Change-Id: I8f518102a0b7cef190cbca59d140d380ae41c326