fix(lsp/java): add false, true and null to method body keywords (#1506)

lsp/java/src/main/java/com/itsaky/androidide/lsp/java/providers/completion/KeywordCompletionProvider.kt

@@ -155,7 +155,10 @@ class KeywordCompletionProvider(
         "long",
         "float",
         "double",
-        "synchronized"
+        "synchronized",
+        "true",
+        "false",
+        "null"
       )
   }
 }