Prevent possible deadlock with LiveRecipientCache.

Thread A: DirectoryHelper#updateContactsDatabase() acquires database lock
Thread B: LiveRecipientCache#getSelf() acquires lock on LiveRecipientCache
Thread A: DirectoryHelper#updateContactsDatabase() calls Recipient.externalContact(), which eventually needs LiveRecipientCache lock
Thread B: Needs to read the database (e.g. line 120) to get information about itself

So A has the DB lock but needs the LiveRecipientCache lock, and B has
the LiveRecipientCache lock but needs the DB lock.

In general, we need to avoid acquiring any new locks in a transaction,
but for now, this specific instance looks like it could be solved by
using a unique lock for LiveRecipientCache#getSelf().

app/src/main/java/org/thoughtcrime/securesms/recipients/LiveRecipientCache.java

@@ -4,6 +4,7 @@ import android.annotation.SuppressLint;
 import android.content.Context;
 
 import androidx.annotation.AnyThread;
+import androidx.annotation.GuardedBy;
 import androidx.annotation.NonNull;
 import androidx.lifecycle.MutableLiveData;
 
@@ -33,11 +34,14 @@ public final class LiveRecipientCache {
   private static final int CACHE_MAX      = 1000;
   private static final int CACHE_WARM_MAX = 500;
 
+  private static final Object SELF_LOCK = new Object();
+
   private final Context                         context;
   private final RecipientDatabase               recipientDatabase;
   private final Map<RecipientId, LiveRecipient> recipients;
   private final LiveRecipient                   unknown;
 
+  @GuardedBy("SELF_LOCK")
   private RecipientId localRecipientId;
   private boolean     warmedUp;
 
@@ -111,7 +115,7 @@ public final class LiveRecipientCache {
   }
 
   @NonNull Recipient getSelf() {
-    synchronized (this) {
+    synchronized (SELF_LOCK) {
       if (localRecipientId == null) {
         UUID   localUuid = TextSecurePreferences.getLocalUuid(context);
         String localE164 = TextSecurePreferences.getLocalNumber(context);