ueventd: allow using external firmware handlers

Userspace may want to load a different firmware than the one that the
kernel requests in some cases, therefore this change adds the ability
to ueventd to run an external handler that will determine the name of
the file that should actually be loaded.

Bug: 138352500
Test: unit tests
Change-Id: Ic5da37268fd78109f83ae52d1b903bf7322a5ee5