SoftAp: Use toString() API in place of toBinaryString for indexes

Security option index values are currently converted using
toBinaryString() which converts indexes to binary String. Thus
index values doesn't match the expected Security config values in
SoftApConfiguration.

This commit uses toString() API to avoid index mismatch.

Bug: 197497991
Test: manual test
atest -c WifiTetherSecurityPreferenceControllerTest

Change-Id: I43d236d071a8141845f312c63b1a693fde254358