Introduce indirection layer on top Cronet ToT and Stable import channels

Export the RELEASE_ENABLE_TOT_CRONET build system flag as a Soong
variable. This way //external/cronet/Android.bp can make use of it (see
http://aosp/3507996).

Test: m
Bug: b:396593141
Merged-In: Icc3afb518264b463662b5a085e390bdfccff6e53
Merged-In: Ia20f6d4212ae7dbd6343b09116133a4bbadf1fff

Change-Id: Ib1ec49f3ccbcfd7b1232c8db658d0c80cb67a59a