Replace illustration for share app offline intro

briar-android/artwork/il_share_app_dark.svg

briar-android/artwork/il_share_app_light.svg

briar-android/artwork/share_app.svg

...arproject/briar/android/hotspot/HotspotIntroFragment.java

briar-android/src/main/res/drawable-night/il_share_app.xml

briar-android/src/main/res/drawable/ic_share_app.xml

briar-android/src/main/res/drawable/il_share_app.xml

briar-android/src/main/res/layout/fragment_hotspot_intro.xml