ueventd: make some FirmwareHandler fields const

This is a little refactoring CL. Add explicit `const` to const
FirmwareHandler fields that are never modified after initialization.
This is useful for readability and makes its methods being thread-safe
more explicit.

Bug: 400592897
Test: Cuttlefish boots
Change-Id: Ie51344b029fe89f883c896dc7a3ec450e3a3b1ec