Add documentation about ART Service.

Bug: 264794495
Bug: 269082327
Test: N/A
Ignore-AOSP-First: ART Services
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:e231149f388b71181dfc6d65c0a531fc36e65aac)
Merged-In: Ia8ab6703b906d3c5232dc6c4acea458de84d2f9c
Change-Id: Ia8ab6703b906d3c5232dc6c4acea458de84d2f9c
1 file changed