UI improvement of network activity indicator on status bar

The activity indicators which can be turned on by a configuration,
shouldShowActivityConfig will jup around in current implementation.

This change will instead use opacity to indicate activity, using
grey when no activity and white when active. This will lead to
the icons keeping the same position all the time.

Bug: 266797961
Test: adb shell settings put global sysui_demo_allowed 1
      adb shell am broadcast -a com.android.systemui.demo
        -e command enter
      adb shell am broadcast -a com.android.systemui.demo
        -e command network -e fully true -e wifi show
        -e ssid demoSsid -e level 3 -e activity in
      adb shell am broadcast -a com.android.systemui.demo
        -e command network -e fully true -e mobile show
        -e level 3 -e activity in
Flag: ACONFIG com.android.systemui.status_bar_static_inout_indicators DEVELOPMENT

Co-Author: Leif Martensson <leif.martensson@sony.com>
Change-Id: Ifc971566799ba22c0f98f86b4fbf3ff47360a3b1