Disable usage of libunwind

The Android blueprint for libunwind disables the target on Darwin,
causing a build failure if we depend on it. We don't need libunwind in
production code so don't emit a dependency on it.

TBR=primiano@google.com
Change-Id: Ic1807927a8524e708292912512e68e141fa4d259
diff --git a/tools/gen_android_bp b/tools/gen_android_bp
index 8c6ea85..4134e5c 100755
--- a/tools/gen_android_bp
+++ b/tools/gen_android_bp
@@ -77,7 +77,8 @@
 
 
 def enable_libunwind(module):
-    module.shared_libs.append('libunwind')
+    # libunwind is disabled on Darwin so we cannot depend on it.
+    pass
 
 
 # Android equivalents for third-party libraries that the upstream project