Remove downgrade check for targetSandboxLevel

This is causing issues with apps adopting Instant Apps and the security
requirements that lead to downgrade being disallowed were either
reverted in late O or moved into target API checks.

Change-Id: Ide3913091788d6a37bf2d17ca22769f6bbc6d3c7
Fixes: 80307033
Test: Install v2 sandbox APK, try and upgrade with a v1 sandbox APK.