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: Ie0d87c44b78333df148937ab5f1fe3cac8e9b752
diff --git a/Android.bp b/Android.bp
index add6ad5..42dae7c 100644
--- a/Android.bp
+++ b/Android.bp
@@ -21,7 +21,7 @@
         // TODO(avakulenko): Android.mk had this for OS version string:
         // "Android $(PLATFORM_VERSION) $(TARGET_ARCH_VARIANT)"
         // but Soong doesn't have access to those config variables yet.
-        "-DOS=\\\"Android\\\"",
+        "-DOS=\"Android\"",
         "-Werror",
         // Bug: http://b/29823425 Disable -Wvarargs for Clang update to r271374
         "-Wno-varargs",