gn2bp: add location tag helper

This is a very common operation.

Test: none
Change-Id: I122fd6bfa4b7fcabc012c8e69050c241083dd8a1
diff --git a/tools/gn2bp/gen_android_bp b/tools/gn2bp/gen_android_bp
index 6b1df3b..c6dccbb 100755
--- a/tools/gn2bp/gen_android_bp
+++ b/tools/gn2bp/gen_android_bp
@@ -649,6 +649,10 @@
     if hasValue:
       self.target.args.pop(i)
 
+  # wrap filename in location tag.
+  def _location_tag(self, filename):
+    return '$(location %s)' % filename
+
   def get_args(self):
     if self.target.script == "//build/write_buildflag_header.py":
       # write_buildflag_header.py writes result to args.genDir/args.output