WifiManager: Defer Async Channel init

Now that WifiManager instances are separate and do not depend
on a single static AsyncChannel for the entire process, all
contexts within a process will create an async channel on init.
Since creating this channel requires special permissions that
some applications do not have, it is problematic for them all
to request it in the process of getting a WifiManager instance.

This CL defers AsyncChannel initialization until it is actually
going to be used.  This allows callers to create WifiManager
instances without necessarily creating an AsyncChannel.
Depending on what parts of the API will be used, some callers
will never need one.  Also, this means that these apps will
not run afoul of permissions exceptions that do not apply to
them unless they actually request functionality that requires
the AsyncChannel.

While here, this CL no longer catches SecurityException while
requesting a messenger from WifiService. That hack appeared to
be an artifact of the static instance of sAsyncChannel -- it
was not clear to WifiManager which context had permissions to
successfully create the messenger, so it was necessary to
silently fail the attempt especially since that attempt was
made in the constructor.  With the restructuring to keep
mAsyncChannel per-instance and now only created at the time
specific API calls require it, a SecurityException is
completely relevant to the caller and should not be discarded.

Bug: 27694933
Test: Boot full image, repro b/27694933 with com.google.android.music

Change-Id: I680bf2443bc5b057e629544a687f985261201ded