Set com_android_systemui_flags_lib sdk_version to current

This change sets com_android_systemui_flags_lib sdk_version to current.
Before the sdk_version is empty, and it will be built against the
implementation of framework jars. Set the sdk_version to system_current
will cut the unnecessary dependency of the implementation of the
framewokr jars. system_current is selected because the generated flag
code needs to use DeviceConfig.

Test: presubmit
Bug: 327965477
Flag: NONE
Change-Id: I75a580a66d789a35aca1389eb505909d64b519f7