gn2bp: Change stem for test targets

* Stem should be the name of the target under GN build system as this is the one expected in the source code
as seen here https://source.chromium.org/chromium/chromium/src/+/main:components/cronet/android/test/javatests/src/org/chromium/net/CronetTestRule.java;l=244?q=loadLibrary%5C(%5C%22cronet_tests&ss=chromium%2Fchromium%2Fsrc

Test: None
Change-Id: I758b9b265d0187c93b4fc8ab48491cc5f7a5a750
diff --git a/tools/gn2bp/gn_utils.py b/tools/gn2bp/gn_utils.py
index bda2754..09a7b80 100644
--- a/tools/gn2bp/gn_utils.py
+++ b/tools/gn2bp/gn_utils.py
@@ -268,6 +268,9 @@
                   'inputs', 'outputs', 'args', 'script', 'response_file_contents', 'ldflags'):
         self._finalize_attribute(key)
 
+    def get_target_name(self):
+      return self.name[self.name.find(":") + 1:]
+
 
   def __init__(self, builtin_deps):
     self.builtin_deps = builtin_deps