floss: add return value for set_ll_privacy

Add return value to set_ll_privacy dbus call.
set_ll_privacy will return false if file permission is wrong.

Bug: 242100561
Tag: #floss
Test: manual run dbus command to enable/disable ll privacy
Test: mma -j32

Change-Id: I4cffeb639615d91c0def8285e8de3bfdbb58904b