Mixed bazel/soong build prototype for genrule

With this change, bazel_module is a specifiable property on
genrule module definitions. With bazel-enabled mode, soong_build will
defer to Bazel for information on these modules.

source build/soong/bazelenv.sh to enter bazel-enabled mode.

Test: Manually verified on bionic/libc genrules using aosp_cf_x86_phone-userdebug
Change-Id: I3619848186d50be7273a5eba31c79989b981d408

android/Android.bp

@@ -15,6 +15,7 @@ bootstrap_go_package {
         "apex.go",
         "api_levels.go",
         "arch.go",
+        "bazel_handler.go",
         "bazel_overlay.go",
         "config.go",
         "csuite_config.go",