Set currView to -1 when doing ViewStack.popAll

Sampler's ViewStack kind didn't correctly set the currView instance
variable to -1 when popping all views.  This didn't afffect the sampler
app, but could affect code that borrowed that kind for their own
implementation.

(bug reported and fix provided by jing-jing.li@hp.com)

Enyo-DCO-1.1-Signed-Off-By: Ben Combee (ben.combee@lge.com)