libsnapshot_fuzzer: Properly unmap images

Instead of unsetting gsid.mapped_images.* directly, properly unmap
them through image manager. This also avoids interference between
different fuzzer runs because the gsid.mapped_images.* properties
are not destroyed in SoftReset.

Test: libsnapshot_fuzzer_test then reboot, device can come up
Fixes: 156689792
Bug: 156380383
Change-Id: I7a198312f63b4b17d8ea96c7df2dd112a910d004