Fix problem where keyguard shows spontaneously

A recent change caused keyguard to ignore the sequence number
when calling doKeyguardLaterLocked().

It also appears we can get into a situation where multiple
requests to show keyguard are pending.  This change ensures
only the latest request gets handled.

Fixes bug 26326350

Change-Id: Ic6cba0054fe235a8838ca8c242b1840c9e8309d6