The Apis in SystemProperties are needed for building ims.apk with
LOCAL_SDK_VERSION := system_current. So @SystemApi is added to
SystemProperties class and methods which are used by vendor apks (i.e.
ims.apk)
Bug: 67726847
Test: 1. build & boot on taimen
2. LOCAL_SDK_VERSION:=system_current in ims.apk && build ims.apk &&
check error count and android_system_stubs_current_intermediates.
Change-Id: I178f8d9b0b1f6bb1455ceec919805c4cc549cb32
5e05a9ae1c