[KV] Extract event reporting and remove traces

Created KeyValueBackupReporter, which includes:
* Logcat (main and event buffers).
* Backup observer.
* Backup manager monitor.

Also removed traces since they weren't very useful. Hopefully this will clear
KVBT of noise and help spot refactor opportunities. Did some small refactors
along w/ it.

Methods are starting to fit in one (big) screen :)

Bug: 110082831
Test: atest KeyValueBackupTaskTest
Test: adb shell bmgr backupnow <kv_package> and verify logs
Test: Kicked-off KV in different scenarios to check logs, this is
      tracked in https://docs.google.com/spreadsheets/d/1EcZ61qapj2cW_Yr99gIlQz0QVJ_BQfVc_1dHBor2cOo/edit?usp=sharing
Test: adb shell bmgr backupnow <kv_package> <fb_package>
      Verify PFTBT is kicked-off and logs
Change-Id: I1486049861543b5ad558b45cf0e0206532e20632