Preparing for work on string interning for AppImages.

* Allow classes that have string fields to be put into AppImages.
* Updated tracing messages to be more descriptive and searchable.
* Added new tracing objects to more accurately report time spent in
handling AppImages.

Test: Built image / flashed device / ran applications

(cherry picked from commit e3015b59fb297d236f0b3ce844eef81c67e423d3)

Change-Id: I6e5fba8c8ddeff8e077672b799d244e767478373
diff --git a/runtime/class_linker.h b/runtime/class_linker.h
index efe29d3..e06a398 100644
--- a/runtime/class_linker.h
+++ b/runtime/class_linker.h
@@ -111,7 +111,7 @@
 
 class ClassLinker {
  public:
-  static constexpr bool kAppImageMayContainStrings = false;
+  static constexpr bool kAppImageMayContainStrings = true;
 
   explicit ClassLinker(InternTable* intern_table);
   virtual ~ClassLinker();