Avoid keyguard UI layout dimensions underflow

KeyguardBouncer could have 0 as its content area size. Two transient
cases found: on device boot with a 0 sized StatusBarWindowView and
first time showing after user switching. Be defensive and protect
against the case.

Bug:21333202
Change-Id: I73eabf64c597ea3ae96ccfba1aecf702da8dfb23