gn2bp: Drop upstream boringssl build rules in favor of local Android.bp

Due to AOSP's FIPS requirements, AOSP-Cronet's BoringSSL must be
built differently than Chromium-Cronet's BoringSSL. The downstream way
of building BoringSSL is manually mantained in
//external/cronet/third_party/boringssl. So, whenever a target depends
on upstream's boringssl translate that to the downstream custom target.

Test: m && CtsNetHttpTestCases
Change-Id: I34a69de79397c35358745388be433934c203d2b2