Increase contrast between identicon and background.

briar-android/res/layout/author_view.xml

briar-android/res/layout/dropdown_author.xml

briar-android/res/layout/list_item_contact.xml

briar-android/res/layout/list_item_msg_in.xml

briar-android/res/values/dimens.xml

...rc/im/delight/android/identicons/AsymmetricIdenticon.java

...roid/src/im/delight/android/identicons/IdenticonBase.java

.../src/im/delight/android/identicons/IdenticonDrawable.java

...src/im/delight/android/identicons/SymmetricIdenticon.java