Remove metrics collector service trampoline

No longer needed to trampoline between RTTI and non-RTTI compiles.

Move the service impl from libmetricscollector service to the
metrics_collector executable, now that RTTI is no longer used (and the impl
is not used by binder clients).

Other fixups, including:
* Move service registration and binder watcher to metrics_collector main.
* Compile libmetricscollectorservice with clang just because.

Bug: 26292405
Change-Id: Iae8115c13b203ff03d49bf7678d5eb3ede30e855