floss: Refactor GATT apis to use GattStatus

Gatt apis were inconsistent between u8 and i32 for representing status.
To make this easier on clients, serialize all GattStatus as u32 and
convert various status values in Gatt to GattStatus.

Bug: 193686094
Tag: #floss
Test: Build for ChromeOS and Linux
Change-Id: Ie058a4ba6f2e6957cf8a1731533f3773c49b20c9