fastbootd: add O_CLOEXEC/O_BINARY for OpenPartition

O_CLOEXEC ensures fastbootd does not leak fds to the parent
process (recovery).

O_BINARY has no effect in Linux, but it is explicit that
the file (partition) to read is a binary file.

Test: pass
Bug: 173654501

Change-Id: Idba922965ce666af1b7ee460ec7449fabd511c35