Use a single thread for DB access from the UI.

The UI may access the DB in response to UI or DB events; to maintain a
consistent view of the DB's contents, the tasks performing these
accesses must be prevented from overlapping, and must produce consistent
results if reordered. A single-threaded executor and latches are used to
prevent tasks from overlapping, without blocking non-UI access to the
DB.

briar-api/src/net/sf/briar/api/android/DatabaseUiExecutor.java

@@ -0,0 +1,18 @@
+package net.sf.briar.api.android;
+
+import static java.lang.annotation.ElementType.FIELD;
+import static java.lang.annotation.ElementType.PARAMETER;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+import com.google.inject.BindingAnnotation;
+
+/**
+ * Annotation for injecting the executor for accessing the database from the UI.
+ */
+@BindingAnnotation
+@Target({ FIELD, PARAMETER })
+@Retention(RUNTIME)
+public @interface DatabaseUiExecutor {}