gn2bp: Add Android.extras.bp support

Test: TH
Change-Id: I66bbbb17446ea1f87e07b3d63d2c4e1ffa9d1a51
diff --git a/tools/gn2bp/gen_android_bp b/tools/gn2bp/gen_android_bp
index 592c070..fd85e7d 100755
--- a/tools/gn2bp/gen_android_bp
+++ b/tools/gn2bp/gen_android_bp
@@ -45,6 +45,8 @@
     '//components/cronet/android:cronet_android_mainline',
 ]
 
+EXTRAS_ANDROID_BP_FILE = "Android.extras.bp"
+
 # Defines a custom init_rc argument to be applied to the corresponding output
 # blueprint target.
 target_initrc = {
@@ -1808,6 +1810,8 @@
 // limitations under the License.
 //
 // This file is automatically generated by %s. Do not edit.
+
+build = ["Android.extras.bp"]
 """ % (tool_name)
   ]
   blueprint.to_string(output)