Move Cronet's MTS/CTS to external/cronet

* Cronet MTS / configuration changes continuously on each new import
that we do and coordinating two different repositories while doing an
import is unnecessarily added friction for which we wish to avoid.
Moving configuration to external/cronet will allow us to reduce test
divergence COMPLETELY which will make our import process  easier.

* The code under test exists under `external/cronet` which makes sense
that the testing modules should live next to the code under test.

* Moved CTS tests to external/cronet as the code under test lives in
external/cronet.

Test: TH
Bug: 304217500
Bug: 338358650
Change-Id: I7537985f66ba36169219472cc268fe8fd5b3c58d
30 files changed