[IncrementalService] getLoadingProgress (v1)

This is to unblock Launcher's work on progress ring. Currently it uses
incfs getFilledBlocks(). Will switch to the new incfs progress reporting
API once it is ready.

Test: unit test
Test: adb shell dumpsys incremental
BUG: 165799231
Change-Id: Icd68124806454f888826294da36f109bca9771ac