Clean up code in BugreportProgressService

* Make fields (progress, lastUpdate and finished) atomic to make them
thread safe and also reduce getters/setters.
* Make lastProgress a private field in BugreportInfo.
* Move deleteScreenshots to BugreportInfo class.
* Make fields with lock protected getters/setters as private.
* Make fields that should not be changed as final.

Bug: 147033613
Test: manual
Change-Id: I8f0fb4865c1b7c5d62bebca3e250eee59b4e71f4