ART: add weighted allocated bytes metrics

`process_cpu_start_time_` is moved from `art::Runtime` to `art::gc:Heap`
so that it could be reset when the app starts (current process is forked
from Zygote).

`process_cpu_end_time_` is removed from `art::Runtime` because it's only
used when the runtime is about to shutdown, so it suffices to capture it
in a local variable.

Test: Run art with -XX:DumpGCPerformanceOnShutdown on some benchmarks.
Bug: 112187497
Change-Id: I154fdb6acdf4e0c21dff835807bd4e2cf311e3d1