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 e3015b59)

Change-Id: I6e5fba8c8ddeff8e077672b799d244e767478373