gn2bp: add helper function to rebase relative dirs

Test: none
Change-Id: I68f8d670e90c70520355c62ae1b868ce0567156a
diff --git a/tools/gn2bp/gen_android_bp b/tools/gn2bp/gen_android_bp
index c6dccbb..16244bd 100755
--- a/tools/gn2bp/gen_android_bp
+++ b/tools/gn2bp/gen_android_bp
@@ -653,6 +653,10 @@
   def _location_tag(self, filename):
     return '$(location %s)' % filename
 
+  # deletes the leading ../../
+  def _rebase_directory(self, filename):
+    filename = re.sub('^\.\./\.\./', '', 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