ART: Remove experimental flags for plugins and agents

Remove experimental flags guarding plugins and agents.

Partial revert of commit 7233c7e752c0d26387d143ee74420e9cd1f09390.
Partial revert of commit 185d134a3b43ab7529053e965917e0fa74bceba4.

Bug: 31455788
Test: m test-art-host
Change-Id: I11a19f24b88ced8da7a28e2cb7a2f602aab884a8
41 files changed