[KV] Cancellation and small refactors

* Refactored cancellation a bit.
* Added tests for cancellation, had to break down handleCancel() in 2 methods.
* Removed BackupRequest.
* Removed onPmFoundInQueue() report.
* Renamed a few methods.
* Moved PM package synthetization to backupPm().

Test: atest RunFrameworksServicesRoboTests
Test: adb shell bmgr backupnow <kv_package>
Test: 1. adb shell bmgr backup <kv_package>
      2. adb shell bmgr run
Test: 1. adb shell bmgr backupnow com.google.android.apps.backupapp
      2. Cancel while in-flight
      3. Verify data not backed-up
Test: 1. adb shell bmgr backupnow --non-incremental com.google.android.apps.backupapp
      2. While in-flight, check blank_state file exists.
      3. When it finishes, check blank_state file does not exist.
Test: atest BackupManagerHostTest
Change-Id: Ie74a19189dc700ef517417b54ecb8930c6aa41ce