Make stopped state a first-class thing

Bug: 27286867

WindowManager has committed to stopped state
controlling the lifecycle of the Surface, so
make that a first-class thing in HWUI as well.

This makes it more resistent to things like
a rogue updateSurface() happening while mStopped=true,
leading to bad things down the line. Instead let
the surface be changed/updated as often as desired,
and just block any attempt to draw on that surface.

Also removes some unnecessary makeCurrent()s, as
EglManager ensures that we *always* have a valid
GL context now (using a pbuffer surface if there is
no window surface set)

Change-Id: Iead78ddebc7997e8fdb0c9534836352f5e54b9bd