Refresh after clearing, tweak warn threshold.

Refresh UI after clearing cache.  Hide user header on single-user
devices to match UX spec.  Refine adopted storage warning threshold
based on experiments with various devices.

For example, a typical internal storage device runs the benchmark
in about 600ms.  Very fast SD cards take about 1700ms, moderate speed
SD cards take about 3800ms, and slow SD cards take about 6800ms.

Bug: 22345902, 22487370
Change-Id: I6066f3b78dbb32277ae94b79e325500724f83c7e