Add non-system overlay flag for usb perm dialog

Add private flag SYSTEM_FLAG_HIDE_NON_SYSTEM_OVERLAY_WINDOWS for
UsbPermissionActivity.

With this flag, UsbPermissionActivity will not been overlapped with
views which are displayed on system alert window

Bug: 183610267
Test: atest UsbPermissionActivityTest#testHideNonSystemOverlay
Test: check the window private flag of UsbPermissionActivity contains
      SYSTEM_FLAG_HIDE_NON_SYSTEM_OVERLAY_WINDOWS or not.
Change-Id: I0863ff347ac3c2300f0dc649a630f55473ee180a