Enable extra compilation warnings

Also, fix existing compilation warnings.

Test: Code compilation
Change-Id: I153a5927ad03a76562185e972bab04b51f499012
diff --git a/Android.bp b/Android.bp
index cf5087e..86d3b14 100644
--- a/Android.bp
+++ b/Android.bp
@@ -12,5 +12,5 @@
     ],
     // -D_32BIT_FIXED_POINT should be added to cflags for devices without a FPU
     // unit such as ARM Cortex-R series or external 32-bit DSPs.
-    cflags: ["-O2", "-Werror"],
+    cflags: ["-O2", "-Werror", "-Wall", "-Wextra"],
 }