__android_log_is_debuggable() shouldn't be an apex symbol

Remove a superfluous check in fs_mgr, since those return values are
already impossible for a non-debuggable build.  Replace a one time
call to __android_log_is_debuggable(), since there's no reason to
cache the value or use this symbol.

Test: build
Change-Id: Icd4bef7b616c49d304303747388d7e3018c6fcfc
diff --git a/liblog/liblog.map.txt b/liblog/liblog.map.txt
index 69a807b..ce4c53c 100644
--- a/liblog/liblog.map.txt
+++ b/liblog/liblog.map.txt
@@ -51,7 +51,7 @@
 LIBLOG_O {
   global:
     __android_log_is_loggable_len;
-    __android_log_is_debuggable; # apex vndk
+    __android_log_is_debuggable; # vndk
 };
 
 LIBLOG_Q {