Relax minimum signature scheme version for apps on system partition

Android 11 requires a minimum V2 APK signature for apps targeting SDK
version 30+; however some apps on a system partition can only be signed
with the V1 signature scheme. This commit relaxes the minimum signature
scheme version to allow for these apps on a system partition.

Bug: 158728035
Test: atest PackageManagerTest
Test: atest PackageManagerTests
Test: atest PkgInstallSignatureVerificationTest
Change-Id: I1a95fd6894cc937e00ad1ac54d1846b51b48e9cd