Null safety cleanups.

...project/briar/android/AndroidNotificationManagerImpl.java

...rg/briarproject/briar/android/activity/BriarActivity.java

...g/briarproject/briar/android/blog/BaseControllerImpl.java

...ava/org/briarproject/briar/android/blog/BlogFragment.java

...ava/org/briarproject/briar/android/blog/FeedFragment.java

...a/org/briarproject/briar/android/blog/ReblogActivity.java

...iarproject/briar/android/contact/ContactListFragment.java

...android/contactselection/BaseContactSelectorFragment.java

...oject/briar/android/conversation/AliasDialogFragment.java

...ject/briar/android/conversation/ConversationActivity.java

...riarproject/briar/android/conversation/ImageFragment.java

...g/briarproject/briar/android/forum/ForumListFragment.java

...ct/briar/android/fragment/ScreenFilterDialogFragment.java

...ct/briar/android/introduction/ContactChooserFragment.java

...g/briarproject/briar/android/keyagreement/CameraView.java

...ar/android/keyagreement/ContactExchangeErrorFragment.java

...ject/briar/android/keyagreement/KeyAgreementActivity.java

...ject/briar/android/keyagreement/KeyAgreementFragment.java

.../briarproject/briar/android/login/AuthorNameFragment.java

...va/org/briarproject/briar/android/login/DozeFragment.java