Upgrade WifiKeystore to @SystemApi.

Previously, these APIs were annoted with
@SystemApi(client = SystemApi.Client.MODULE_LIBRARIES).

Bug: 324601616
Bug: 296077015
Test: Manual test - create local shell command to
      write/read to WifiKeystore. Verify that
      all operations work as expected.
Change-Id: I59259019ad631472a87ebd6bdad3bced983ac8ef