__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