fastbootd: Refactor for getvar all.

It is easier to implement getvar all if we can invoke each variable
handler and collect their results. This change reverts the handlers to
return their messages as an outparam, rather than going through
FastbootDevice.

Bug: 78793464
Test: fastboot getvar works
Change-Id: I8544251ce517526b26435526756359ce220520cc
3 files changed