Replace illustration for mailbox onboarding

briar-android/artwork/il_mailbox_dark.svg

briar-android/artwork/il_mailbox_light.svg

briar-android/artwork/mailbox_onboarding_dark.svg

briar-android/artwork/mailbox_onboarding_light.svg

...riarproject/briar/android/mailbox/SetupIntroFragment.java

...oid/src/main/res/drawable-night/ic_mailbox_onboarding.xml

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

...r-android/src/main/res/drawable/ic_mailbox_onboarding.xml

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

...roid/src/main/res/layout/fragment_mailbox_setup_intro.xml