libfs_avb: support rollback protection for Live GSI

This commit extracts the security patch level (SPL), e.g.,
com.android.build.system.security_patch = 2019-04-05 from AVB property
descriptors when attempting to mount a standalone image (e.g., live
GSI). Then compares the SPL between the old system.img and the new live
system.img for rollback protection.

Bug: 122705329
Test: boot an old Live GSI, checks rollback is detected
Change-Id: I7aae58c0b2062a3ff57ed932ad58e7b604453fed