Implement support library API generation and check in Gradle (again)

A subsequent CL will remove the individual API check targets and switch
the update-support-api make target over to invoke the Gradle target.

Bug: 28124434
Change-Id: I3dbf7f4945985fa48fb12b004e9e36ba6212abe4
diff --git a/.gitignore b/.gitignore
index e1876f9..6c671f7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -8,4 +8,4 @@
 **/gen
 *.iml
 **/out
-buildSrc/
+buildSrc/build