Speed up verified methods

Switch to an array of atomic pointers instead of a map. Removes lock
and map lookup. Also address comments from previous CL.

GetVerifiedMethod: 1.59% -> 0.18% of compilation time.

Install time seems to goes down by around 1%.

Also has significant RAM savings (FB host compile):
dex2oat native alloc: 84695472B -> 71268736B

For the JIT case, the old method is used to prevent any increase in
RAM usage.

Bug: 32641252

Test: test-art-host
Change-Id: I47b4b8a4a3cb3f8ef23e36a888b8885e12168787