fastbootd: allow passage of flags to open partition

Allow caller to pass additional flags when opening a partition. Obsolete
usage of boolean read, and make previous callers use O_RDONLY instead.
Explicitly OR (O_EXCL | O_CLOEXEC | O_BINARY) to keep existing design
working as is.

Test: flash locally and reach home screen
Bug: 205151372
Signed-off-by: Konstantin Vyshetsky <vkon@google.com>
Change-Id: I48fbca459a17fcf0b0926ab339585e3bd8e31e35