Create gn arg templates for use with blimp builds. Update build doc.

build/args/blimp_client.gn

Index: build/args/blimp_client.gn
diff --git a/build/args/blimp_client.gn b/build/args/blimp_client.gn
new file mode 100644
index 0000000000000000000000000000000000000000..a072eab57e3e38f23058492d2187b4115a6da84a
--- /dev/null
+++ b/build/args/blimp_client.gn
@@ -0,0 +1,11 @@
+# GN args template for blimp Android client.
+#
+# Add import to arg.gn in out directory and run gn gen on the directory to use.
+# E.g. for out directory out/foo:
+# echo "import(\"build/args/blimp_client.gn\")"  > out/foo/args.gn
+# gn gen out/foo
+#
+# Use gn args to add your own build preference args.
+
+target_os = "android"
+is_component_build = true