Update logic for detecting pre-apexd services to check that the ART APEX is

mounted.

Necessary to make e.g. bootanimation work, which depends on (at least)
libandroidicu.so in the ART APEX.

Test: Build & boot
Bug: 135753770
Exempt-From-Owner-Approval: Approved internally
Change-Id: Ibab4f5f7a243d0815b9c70a537bae8d77dee2fbb
Merged-In: Ibab4f5f7a243d0815b9c70a537bae8d77dee2fbb