Migrate CTS_SYSTEM_API_CHANGEID to @EnabledSince

The actual target sdk for this changeid is currently irrelevant; it does
need to be larger than the target SDK of the CTS test app (to allow
opting in), and invisible in dev UI. The latter was already the case,
however made it explicit.

Bug: 149103556
Bug: 149469819
Test: adb shell dumpsys platform_compat | grep CTS_SYSTEM_API_CHANGEID
Change-Id: I64a10bb0964349eb212d26cf40e9f15ad2ba61da