Update escaping in Android.bp files

Strings like cflags in Android.bp files are parsed by blueprint,
written to build.ninja files, parsed by ninja, and then passed to
/bin/sh -c.  This had resulted in a combination of blueprint
(\"), ninja ($$), and shell (\$) escaping being necessary.

Soong has been updated to automatically handle ninja and shell
escaping, remove extra escaping from Android.bp files.

Bug: 31221587
Test: m -j
Change-Id: Ib1deac0a236a2ad37fce394bbdefbf5a781a38a7
diff --git a/audio_utils/Android.bp b/audio_utils/Android.bp
index c198d7a..969ed07 100644
--- a/audio_utils/Android.bp
+++ b/audio_utils/Android.bp
@@ -45,7 +45,7 @@
             ],
         },
         host: {
-            cflags: ["-D__unused='__attribute__((unused))'"],
+            cflags: ["-D__unused=__attribute__((unused))"],
         },
     },
 }