ART: Add very-large threshold to dex2oat

ART: Add very-large threshold to dex2oat

Add a variable threshold to dex2oat. If the total dex file size for
an app reaches this threshold, dex2oat will punt all compilation and
compile the app with verify-at-runtime. This ensures smaller compile
time and memory thrashing, while still extracting the dex files and
thus helping with dirty memory later.

Added tests.

Bug: 29557002
Bug: 29790079
Test: m test-art-host-gtest-dex2oat_test
Change-Id: I78870e4a80ccaafcbbe56839e61ced0acd2ca05e