Fix crashes after HDMI disconnect

The display was being removed from SurfaceFlinger's list before we
had a chance to reset HWComposer's layer list, so we were passing
stale data into the hardware composer (which has its own per-display
data).  This resulted in "invalid gralloc handle" complaints.
We now clear the layer list immediately after removing the display.

The display was being removed while its EGLSurface was still
"current", resulting in "cancelBuffer: BufferQueue has been
abandoned" complaints.  We now call makeCurrent on the primary
display before removing the external display.

Bug 7274254

Change-Id: Ia59e3a61d7ec46488b96bf93ec5e4ed3488b70e4