fastbootd: use O_DIRECT for write partition

Direct writes for partition flashing significantly increase performance.
Use O_DIRECT flag when opening partition for flashing. Additionally use
a 4096b aligned buffer which is required for O_DIRECT.
Switch from using 8MB write buffer to 1MB write buffer, as the extra
allocation has no performance impact.

Test: flash locally and reach home screen
Bug: 205151372
Signed-off-by: Konstantin Vyshetsky <vkon@google.com>
Change-Id: I060f438cf698d0fda1e59e35338bb5dc1cd05b51