Expand SystemApi annotation for generating system api

Metalava apparently normalizes the SystemApi annotation that was
previously used to the form that this change uses so this is
effectively a no-op. However, this does make this code more
self-consistent and also matches the form that is used in the framework
defaults for the mainline modules.

Test: m checkapi
Bug: 155164730
Change-Id: Ic99b7333730ed8b918d8af72ae7bd6a851c2093a