gae-interop-testing: add GAE interop tests (#3535)

See `gae-interop-testing/README.md` for details on how to run the tests.
diff --git a/settings.gradle b/settings.gradle
index dbe8ab7..780e8c6 100644
--- a/settings.gradle
+++ b/settings.gradle
@@ -12,6 +12,8 @@
 include ":grpc-testing"
 include ":grpc-testing-proto"
 include ":grpc-interop-testing"
+include ":grpc-gae-interop-testing-jdk7"
+include ":grpc-gae-interop-testing-jdk8"
 include ":grpc-all"
 include ":grpc-benchmarks"
 include ":grpc-services"
@@ -29,6 +31,8 @@
 project(':grpc-testing').projectDir = "$rootDir/testing" as File
 project(':grpc-testing-proto').projectDir = "$rootDir/testing-proto" as File
 project(':grpc-interop-testing').projectDir = "$rootDir/interop-testing" as File
+project(':grpc-gae-interop-testing-jdk7').projectDir = "$rootDir/gae-interop-testing/gae-jdk7" as File
+project(':grpc-gae-interop-testing-jdk8').projectDir = "$rootDir/gae-interop-testing/gae-jdk8" as File
 project(':grpc-all').projectDir = "$rootDir/all" as File
 project(':grpc-benchmarks').projectDir = "$rootDir/benchmarks" as File
 project(':grpc-services').projectDir = "$rootDir/services" as File