SF: Compile with -Wextra

Compile with -Wextra for additional conversion checks. In each file
which produces compile errors -Wextra is ignored.

Bug: 175126758
Test: m surfaceflinger && m libsurfaceflinger_unittest
Change-Id: Iec6bcc699715a99c8c65d891ab3c0481e884c728
diff --git a/services/surfaceflinger/Android.bp b/services/surfaceflinger/Android.bp
index fdb8aaf..a7cd258 100644
--- a/services/surfaceflinger/Android.bp
+++ b/services/surfaceflinger/Android.bp
@@ -3,6 +3,7 @@
     cflags: [
         "-Wall",
         "-Werror",
+        "-Wextra",
         "-Wformat",
         "-Wthread-safety",
         "-Wunused",