[android] StartupActivity: Address first round of review comments

...java/org/briarproject/briar/android/BriarApplication.java

...ain/java/org/briarproject/briar/android/BriarService.java

...rg/briarproject/briar/android/StartupFailureActivity.java

...org/briarproject/briar/android/account/SetupActivity.java

...riarproject/briar/android/activity/ActivityComponent.java

...riarproject/briar/android/login/OpenDatabaseFragment.java

...org/briarproject/briar/android/login/StartupActivity.java

...briarproject/briar/android/settings/SettingsFragment.java

...iarproject/briar/android/splash/SplashScreenActivity.java

...org/briarproject/briar/android/test/TestDataActivity.java