Use -Werror in frameworks/rs

* Use -Wno-error= to keep existing warnings, to fix later

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I1cbfb56740618ee9c78d82218ae49931251689e7

cpp/Android.bp

diff --git a/cpp/Android.bp b/cpp/Android.bp
index e25e39a..710c690 100644
--- a/cpp/Android.bp
+++ b/cpp/Android.bp
@@ -6,6 +6,8 @@
     include_dirs: ["frameworks/rs"],
 
     cflags: [
+        "-Wall",
+        "-Werror",
         "-Wno-unused-parameter",
         "-std=c++11",
     ],