Avoid resuming activity before launch.

The locktask logic would always bring a locked task to the front and
then resume it when locking. When a task is to be locked at launch that
would cause it to resume immediately before onLaunch was called. Which
would cause havoc because the token was not yet in
ActivityThread.mActivities. This lead to premature finish() calls and
looping restarts.

This change causes the resume to only be called when an app calls
startLockTask. Otherwise the resume call is skipped.

Plus additional locktask debug logging.

Fixes bug 21031298.

Change-Id: I756b0d607827d0ec7a123377db04d9377c41776d