Report GC collector type with ART event/value metrics

After this change, the Runtime will report the Garbage Collector
type in use, to help assessing the performance of the new CMC
garbage collector based on userfaultfd.

Ignore-AOSP-First: this topic includes changes in the atoms.proto file,
for which AOSP and internal repo have diverged, causing merge conflicts
while running presubmits.

Bug: 249772734
Test: atest ArtGtestsTargetChroot
Change-Id: I51004d6ab0fd208c1d9a639856ee38f942c937e9
 (cherry picked from commit I725078590e864bd2a3d9d67563e4e43921248622)
Merged-In: I51004d6ab0fd208c1d9a639856ee38f942c937e9
2 files changed