pread() data into memory instead of lazily mmap everything am: b936892050 am: a949d23e78 am: 41f476a8ec

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2030887

Change-Id: If7e6c83a1af225e01d2a5930a2faf3b18bc653d6