libsnapshot: Add script to test snapshot updates

$time ./system/core/fs_mgr/libsnapshot/apply-update.sh --update-static-partitions

Update completed

real	1m19.850s
user	0m10.319s
sys	0m7.448s

Dynamic parition update only:

$time ./system/core/fs_mgr/libsnapshot/apply-update.sh

Update completed

real	0m19.032s
user	0m10.480s
sys	0m7.462s

Bug: 329255363
Test: apply-update.sh

Change-Id: I2af4fe022e35b2b813ff83e2084d22295b3057dc
Signed-off-by: Akilesh Kailash <akailash@google.com>