Do update signal icon for -1 level in AddAppNetworksFragment

The level of a WifiEntry may be -1 (WIFI_LEVEL_UNREACHABLE) due to race
conditions. Thus, only update the signal icon if the level is a valid
value [0, 4]

Bug: 201488249
Test: m
Change-Id: I87f1c108d262bb5a7575c9ad434d2cfb953134af
(cherry picked from commit 9d66b22ee045637f7e331971e20f13d18b3435f0)
1 file changed