app cosmetic changes - wording/icon

[nanometrenat]

Serval Chat App:

• app icon (light grey logo on white background) is difficult to see on phone app screen, need to change to something with more contrast so it's easier to spot
• Keep Alive wording in notification tray ("We have a network, don't kill us Android") needs to be changed to remove all reference to "kill", as is currently somewhat disconcerting. If a notification is needed at all, perhaps it could say something like "We have a network, don't disconnect us..."

[lakeman]

Wording changed in f4987b8