Remove unused content observer from AssistManager.

Since I45e58de5f48e3769f114ff830d62c287d2b04d4f we always look up
Settings.Secure.ASSISTANT just before the assist activity is launched.
Hence we don't need to monitor following runtime data anymore.
 - changes of Settings.Secure.ASSISTANT with a content observer
 - switches of the current user id

The primary goal of this CL is to make it clearer that we are lazily
resolving the content of Settings.Secure.ASSISTANT by removing
unnecessary code.  No user visible behavior change is intended.

Bug: 23097413
Change-Id: I079807db761c5543ba666c427d9b074794becf6e