liblog: add android_log_write_string8_len

android_log_write_string8_len(android_log_context ctx,
                              const char *value, size_t maxlen)

Caps the supplied string to a maxlen length. Alter API to handle
a NULL pointer for the value string for this and
android_log_write_string8() and instead of returning -EINVAL,
act like a null string "" was supplied to preserve the list
location. API is also changed to report the number of characters
actually placed into the android_log_context.

Bug: 27356456
Bug: 19235719
Change-Id: I6a03d405eac1d741555dd05555513ec691e7a46e