ui: inform user of lost network connectivity

...in/java/cx/ring/fragments/AccountsManagementFragment.java

...app/src/main/java/cx/ring/fragments/CallListFragment.java

...in/res/drawable-hdpi/ic_network_disconnect_black_24dp.png

...in/res/drawable-ldpi/ic_network_disconnect_black_24dp.png

...in/res/drawable-mdpi/ic_network_disconnect_black_24dp.png

...n/res/drawable-xhdpi/ic_network_disconnect_black_24dp.png

.../res/drawable-xxhdpi/ic_network_disconnect_black_24dp.png

...res/drawable-xxxhdpi/ic_network_disconnect_black_24dp.png

ring-android/app/src/main/res/layout/frag_accounts_list.xml

ring-android/app/src/main/res/layout/frag_call_list.xml

ring-android/app/src/main/res/layout/item_account_pref.xml

ring-android/app/src/main/res/values/strings.xml