liblog: add __android_log_is_loggable checking to writer

Add __android_log_is_loggable() checking for all buffers except
LOG_ID_SECURITY. Return -EPERM if blocked. Since we are sniffing
the log tag, check validity and return -EINVAL.

NB: Try not to call __android_log_is_loggable() in native code within
a signal handler. Both here, and in the system properties, there
are locking paths that are not guaranteed to play well in that
environment. This has also been the case for the log writer path
even before this change. All attempts have been made to use trylock,
and to use a more expensive code path when contention occurs rather
than lead to deadlock.

Bug: 19544788
Bug: 26178938
Change-Id: I98738c662f6328189a6703251eb8721a05e956f9