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

Change-Id: I1b85f963b514617532ac6ec68dc3851a6625f072