Allow swiping between OnboardingFragments

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

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

...rg/briarproject/mailbox/android/ui/OnboardingViewModel.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

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

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

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