This reverts commit b5aedb30 and relands #17712.

Fixes flutter/flutter#53288 and flutter/flutter#41654.

Together with #17791, this reland addresses some of Jim's concerns in the original PR #17712.

The major part of this PR is still the same as the original PR, and the performance / golden image impacts should be the same.

Reland "Bump simulator version in IosUnitTests & scenario app in preparation for luci xcode 11 migration (#15316)", Reverted in #15313

This reverts commit 8ed688f7.

Revert "Bump simulator version in IosUnitTests & scenario app in preparation for luci xcode 11 migration (#15154)" (#15313)

This reverts commit 3a9121b9.

Bump simulator version in IosUnitTests & scenario app in preparation for luci xcode 11 migration (#15154)