pw_log_android: Make PW_LOG_LEVEL_FATAL fatal again

Since https://pwrev.dev/295040, invoking
PW_HANDLE_LOG(PW_LOG_LEVEL_FATAL, ...) would no longer abort the
process, and would continue executing.

Making matters even worse, the pw_assert_log implementation of
PW_HANDLE_CRASH() follows that up with a dubious PW_UNREACHABLE, which
means that the compiler likely stopped emitting code at that point (in
the branch or function). As a result, execution continues at whatever
arbitrary code happens to follow! If you're lucky, this results in a
SIGSEGV.

This corrects the problem by routing PW_LOG_LEVEL_FATAL to
__android_log_assert() which properly aborts (and is even annotated as
noreturn).

This also replaces the constexpr convert_pigweed_to_android_log_level()
function with a 'static inline' function, since PW_LOG should be usable
in C. In the common case of a constant log level, the compiler should
still be able to optimize it away.

Finally, ths changes the conversion default from ANDROID_LOG_DEBUG to
ANDROID_LOG_WARN. This should never be used since we handle all current
PW_LOG_LEVEL_* values, but ensures that such a log is visible in case a
different level is added.

Test: Verified (`logcat -s DEBUG`) that processes now crash with SIGABRT
  at the intended location.
Bug: 543566936
Change-Id: Idd06077f0f4570d868ad0971fbaad84a986a558a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/452252