hwui: add capability to skip fence sync

Enable via: TARGET_DOESNT_USE_FENCE_SYNC := true
libhwui: skip sync/flush in FlushLayerUpdates when fence sync is disabled
Re-add legacy soft-ap for kitkat (1/2)
LegacyUsbDeviceManager: refresh code based on latest UsbDeviceManager

This should solve various issues with legacy USB stacks, including
tethering.