Introduce indirection layer on top Cronet ToT and Stable import channels

Add a new build system flag that select whether AOSP will use Cronet
from the ToT or Stable import channel. This flag will solely be used
within //external/cronet/Android.bp to select which version of Cronet
will be exposed to the rest of AOSP (see http://aosp/3507996).

Test: m
Bug: b:396593141
Change-Id: Ice998c44041d4eb824de111b348011ddb48affe6