Define sane default target SDK for system_server.

Normal app launches always define a target SDK, but system_server
doesn't pass in an "--target-sdk-version" option, meaning we left
the default value target SDK of "0" in place, which isn't a valid
SDK level, meaning we treat it as targeting Android 1.0.

The majority of target SDK checks are using the higher-level value
provided from Context, meaning they've always been enforced; this
change means that we're now applying lower-level target SDK checks
that go through VMRuntime.

Test: builds, boots, typical operations still work
Bug: 72331067
Change-Id: I6940bb8c3a1cd2234bcacadbf60c902625876a76