Implement Onboarding UI

mailbox-android/build.gradle

mailbox-android/src/main/AndroidManifest.xml

...java/org/briarproject/mailbox/android/MailboxViewModel.kt

...ain/java/org/briarproject/mailbox/android/MainActivity.kt

...org/briarproject/mailbox/android/ui/OnboardingActivity.kt

...org/briarproject/mailbox/android/ui/OnboardingFragment.kt

mailbox-android/src/main/res/anim/enter.xml

mailbox-android/src/main/res/anim/exit.xml

mailbox-android/src/main/res/anim/pop_enter.xml

mailbox-android/src/main/res/anim/pop_exit.xml

mailbox-android/src/main/res/drawable/ic_circle.xml

...-android/src/main/res/layout-land/fragment_onboarding.xml

mailbox-android/src/main/res/layout/activity_main.xml

mailbox-android/src/main/res/layout/activity_onboarding.xml

mailbox-android/src/main/res/layout/fragment_onboarding.xml

mailbox-android/src/main/res/navigation/nav_main.xml

mailbox-android/src/main/res/navigation/nav_onboarding.xml

mailbox-android/src/main/res/values-night/themes.xml

mailbox-android/src/main/res/values/attrs.xml

mailbox-android/src/main/res/values/colors.xml