libsnapshot: Use two threads to run compression

Compression is a hot function in the install path. Use
two threads for compression.

By default, number of thread is set to 1. If the property,
"ro.virtual_ab.compression.threads" is true, the number
of threads is increased to 2.

OTA install time (without post-install) on Pixel 6 Pro with 2 threads:

	  Without-this-patch       With-this-patch

Full OTA: 23 Minutes               17 Minutes

Bug: 254188450
Test: Full/Incremental OTA on Pixel
Change-Id: I4a11dca3a5ebfe11dcc7f0d882332d491f2d7933
Signed-off-by: Akilesh Kailash <akailash@google.com>