Optimize zstd compression

reuse the same context for zstd compression. One context should be used
per compression thread for optimal performance. Discussion and
reccomendations can be found at go/android-hw-compression-vendor

Results (level 3 compression)
full ota on raven with optimizations:  1191.304 seconds
without optimizations:  1461.854 seconds

compression ratio remains unchanged and merge time difference are
negligible ~1% delta (probably just noise)

Test: ota_from_target_files, update_device.py
Change-Id: I3feede9f1f119874e369c54b29c594475fbf7376