recovery: Fork a process for fuse when sideloading from SD card.

For applying update from SD card, we used to use a thread to serve the
file with fuse. Since accessing through fuse involves going from kernel
to userspace to kernel, it may run into deadlock (e.g. for mmap_sem)
when a page fault occurs. Switch to using a process instead.

Bug: 23783099
Bug: 26313124
Change-Id: Iac0f55b1bdb078cadb520cfe1133e70fbb26eadd