don't clear the framebuffer when the framebuffer is not used
don't clear the framebuffer when the framebuffer is not used

when areas of the FB are undefined (transparent windows on top of
nothing), we clear those areas before composition.
however, it makes no sense to do this when the FB is not in use
(case where hwc handles all layers)

Bug: 5360529
Change-Id: If51bb669307e8419bbe1f3a89d1c88e0ec1f216c

xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base