SurfaceFlinger: Enable -WConversion in Android.bp

Enable global -WConversion in Android.bp and turn it off in
individual files. This is the first step to enable -WConversion
in SurfaceFlinger.

Test: boot
Bug: 129481165
Change-Id: Ia752df39ef380bde71705a32582d618242c466f7
diff --git a/services/surfaceflinger/Android.bp b/services/surfaceflinger/Android.bp
index 4c5e5da..8fa35d8 100644
--- a/services/surfaceflinger/Android.bp
+++ b/services/surfaceflinger/Android.bp
@@ -7,6 +7,7 @@
         "-Wthread-safety",
         "-Wunused",
         "-Wunreachable-code",
+        "-Wconversion",
     ],
 }