Add tests, refactor min/targetSdkVersion checks in PackageParser

Tests were added and verified before refactoring to ensure no behavior
changes. This is the first step in revising compat checks for O policy.

Bug: 33455703
Test: PackageParserTest#testComputeMinSdkVersion
Test: PackageParserTest#testComputeTargetSdkVersion
Change-Id: I483b4d23945183702b7e077fa1bd6d3ab5065acc