fuse_sideload: Change the minimal block size to 4096.

run_fuse_sideload() is passing the block size as the max_read
option, so it will only handle a request that involves at most two
blocks at a time. However, the minimal allowed value was set to 1024
prior to this CL, which is inconsistent with the kernel code
(fs/fuse/inode.c) that sets it to the greater of 4096 and the passed-in
max_read option. This would fail the calls with a block size / max_read
less than 4096 due to the wrongly computed block indices.

Note that we didn't observe real issue in practice, because we have been
using 64 KiB block sizes for both of adb and sdcard sideload calls. The
issue only shows up in my local CL (to come later) that uses 1024 block
size in run_fuse_sideload() tests.

Test: recovery_component_test
Test: adb sideload with the new recovery image on angler
Change-Id: Id9f0cfea13d0d193dcb7cd41a1553a23739545f2