DSU installation service: Add event log tags

Log important events to the 'events' buffer so other tools can robustly
and progmatically parse the DSU logs.

Bug: 224909776
Test: adb logcat -b events -v descriptive \
  '*:S dsu_progress_update dsu_install_complete dsu_install_failed'
Change-Id: Iac8d3ec31f51eb9410eb631487e26b4ccb0ccf81