liblog: add -Wall,-Wextra and do related clean-up.

Test: build
Change-Id: I3b33f2b896cb8bf7ad9edf110d1739d559028282
diff --git a/liblog/Android.bp b/liblog/Android.bp
index c38d8cd..588089b 100644
--- a/liblog/Android.bp
+++ b/liblog/Android.bp
@@ -108,7 +108,9 @@
     },
 
     cflags: [
+        "-Wall",
         "-Werror",
+        "-Wextra",
         // This is what we want to do:
         //  liblog_cflags := $(shell \
         //   sed -n \