liblog: add log/log_read.h

Move log reading definitions to their own home.

struct logger_entry
struct logger_entry_v2
struct logger_entry_v3
struct logger_entry_v4
LOGGER_ENTRY_MAX_PAYLOAD
LOGGER_ENTRY_MAX_LEN
struct log_msg
android_logger_get_id()
android_logger_clear()
android_logger_get_log_size()
android_logger_get_log_readable_size()
struct logger_list
android_logger_get_statistics()
android_logger_get_prune_list()
android_logger_set_prune_list()
android_logger_list_alloc()
android_logger_list_alloc_time()
android_logger_list_free()
android_logger_list_read()
android_logger_open()
android_logger_list_open()

Deal with a possible scenario where fcntl.h was not included at the
top of the source files, and log/log_read.h inclusion of fcntl.h
conflicts with a source file inclusion of sys/cdefs.h definition of
__unused macro.

Test: gTest liblog-unit-tests --gtest_filter=\
      liblog.__android_log_write__android_logger_list_read:\
      liblog.android_logger_get_
Bug: 34250038
Change-Id: Ib33544171563d6a351164754c254c4385686025b