[SystemUI] 1.Remove the unnecessary log 2. update signal icon in bg thread

1. The log should be controlled by the DEBUG, otherwise it will be printed infinitely and the performance will be very slow.
The log can be removed after checking, so I removing it instead of adding the DEBUG tag.

2. Do too much get icon in the main thread, move it into bg thread

3. It is unnecessary that update the mobile data layout from the callback, onAccessPointsChanged, because it is used for Wi-Fi.

Bug: 201200079
Test: manual and Q verified pass
Change-Id: I5f4623f9ffb68fae6ca377625aaa039f0f33dd38