Save and restore UI state when screen is rotated.

...-android/src/net/sf/briar/android/HomeScreenActivity.java

briar-android/src/net/sf/briar/android/SetupActivity.java

...src/net/sf/briar/android/blogs/ConfigureBlogActivity.java

...id/src/net/sf/briar/android/blogs/CreateBlogActivity.java

...src/net/sf/briar/android/blogs/WriteBlogPostActivity.java

...c/net/sf/briar/android/groups/ConfigureGroupActivity.java

.../src/net/sf/briar/android/groups/CreateGroupActivity.java

...c/net/sf/briar/android/groups/WriteGroupPostActivity.java

...net/sf/briar/android/identity/CreateIdentityActivity.java

...c/net/sf/briar/android/invitation/AddContactActivity.java

.../src/net/sf/briar/android/invitation/CodeEntryWidget.java

...f/briar/android/messages/WritePrivateMessageActivity.java