Use a per-thread VerifierDeps.

Avoid lock contention on a singleton VerifierDeps by allocating
temporary per-thread VerifierDeps that get merged after verification.

This saves around ~35% compile-times on interpret-only.

Only the creation of extra strings is guarded by a lock, for simplicity.

Test: test-art-host, test-art-target
bug: 32641252
bug: 30937355

Change-Id: I11a2367da882b58e39afa7b42cba2e74a209b75d