Improve conversation screen graphics with less padding

briar-android/res/drawable-hdpi/alerts_and_states_error.png

...r-android/res/drawable-hdpi/message_notification_icon.png

briar-android/res/drawable-hdpi/msg_in.9.png

briar-android/res/drawable-hdpi/msg_out.9.png

briar-android/res/drawable-hdpi/navigation_accept.png

briar-android/res/drawable-hdpi/notice_in.9.png

briar-android/res/drawable-hdpi/notice_out.9.png

...r-android/res/drawable-ldpi/message_notification_icon.png

briar-android/res/drawable-mdpi/alerts_and_states_error.png

...r-android/res/drawable-mdpi/message_notification_icon.png

briar-android/res/drawable-mdpi/msg_in.9.png

briar-android/res/drawable-mdpi/msg_out.9.png

briar-android/res/drawable-mdpi/navigation_accept.png

briar-android/res/drawable-mdpi/notice_in.9.png

briar-android/res/drawable-mdpi/notice_out.9.png

briar-android/res/drawable-xhdpi/alerts_and_states_error.png

...-android/res/drawable-xhdpi/message_notification_icon.png

briar-android/res/drawable-xhdpi/msg_in.9.png

briar-android/res/drawable-xhdpi/msg_out.9.png

briar-android/res/drawable-xhdpi/navigation_accept.png