Add support for android_app_certificate modules

Some android_app modules need certificates located outside their
directory.  Instead of requiring paths from the root of the tree,
add an android_app_certificate module that exports the certificate
files.

Test: m checkbuild
Change-Id: Icbf3898894f3eb857e2d907e3e58dd072c6fabe9
diff --git a/java/java.go b/java/java.go
index 7ef3626..c5414f4 100644
--- a/java/java.go
+++ b/java/java.go
@@ -377,6 +377,7 @@
 	frameworkApkTag  = dependencyTag{name: "framework-apk"}
 	kotlinStdlibTag  = dependencyTag{name: "kotlin-stdlib"}
 	proguardRaiseTag = dependencyTag{name: "proguard-raise"}
+	certificateTag   = dependencyTag{name: "certificate"}
 )
 
 type sdkDep struct {
@@ -797,7 +798,11 @@
 		tag := ctx.OtherModuleDependencyTag(module)
 
 		if _, ok := tag.(*jniDependencyTag); ok {
-			// Handled by AndroidApp.collectJniDeps
+			// Handled by AndroidApp.collectAppDeps
+			return
+		}
+		if tag == certificateTag {
+			// Handled by AndroidApp.collectAppDeps
 			return
 		}