softap: Make SoftApConfiguration.BAND_DUAL systemApi.

SoftApConfiguration.BAND_DUAL need to be accessed from Settings or
other priviledged application. This Commit makes BAND_DUAL @SystemApi

CRs-Fixed: 2642410
Change-Id: I8bd9fd0cc00125569b5befd2e7d4d4f06270350c
4 files changed