Rename enableAutoJoinWhenAssociated() to setEnableAutoJoinWhenAssociated()

This renaming better reflects how this method is a setter, and
supports variable renaming taking place in WifiConfigManager and
WifiStateMachine.

BUG: 28088867
Change-Id: Ic2f7a1f53ffec4072e556d2831bfdcff9fef1ad7
TEST: 'runtest frameworks-wifi' passes.