Don't override pre-release target sdk

If a package targets a pre-release SDK [eg a letter version] it should not
be allowed to be upgraded by a release SDK [eg a number version]. If one
absolutely must upgrade to a release SDK, use the "--force-sdk" option
during install.

Bug: 28345311
Change-Id: Ic9fb209968e7c5da2c80c5ca4c0f44f5125f610a