[Android] libcutils - native 获取/设置 property

前言：

Android 的property系统类似于linux的环境变量，但是更加精细。可以通过adb 设置和读取 property，同时也可以在代码 (JAVA/C++/C) 中设置和获取属性。这有助于我们在运行时控制代码执行逻辑。比如打开 测试开关 或者 dump源数据文件。

工程关联：

库文件：libcutils

头文件：#include <cutils/properties.h>

使用：

/* property_get: returns the length of the value which will never be
** greater than PROPERTY_VALUE_MAX - 1 and will always be zero terminated.
** (the length does not include the terminating zero).
**
** If the property read fails or returns an empty value, the default
** value is used (if nonnull).
*/
int property_get(const char* key, char* value, const char* default_value);/* property_get_bool: returns the value of key coerced into a
** boolean. If the property is not set, then the default value is returned.
**
* The following is considered to be true (1):
**   "1", "true", "y", "yes", "on"
**
** The following is considered to be false (0):
**   "0", "false", "n", "no", "off"
**
** The conversion is whitespace-sensitive (e.g. " off" will not be false).
**
** If no property with this key is set (or the key is NULL) or the boolean
** conversion fails, the default value is returned.
**/
int8_t property_get_bool(const char *key, int8_t default_value);/* property_get_int64: returns the value of key truncated and coerced into a
** int64_t. If the property is not set, then the default value is used.
**
** The numeric conversion is identical to strtoimax with the base inferred:
** - All digits up to the first non-digit characters are read
** - The longest consecutive prefix of digits is converted to a long
**
** Valid strings of digits are:
** - An optional sign character + or -
** - An optional prefix indicating the base (otherwise base 10 is assumed)
**   -- 0 prefix is octal
**   -- 0x / 0X prefix is hex
**
** Leading/trailing whitespace is ignored. Overflow/underflow will cause
** numeric conversion to fail.
**
** If no property with this key is set (or the key is NULL) or the numeric
** conversion fails, the default value is returned.
**/
int64_t property_get_int64(const char *key, int64_t default_value);/* property_get_int32: returns the value of key truncated and coerced into an
** int32_t. If the property is not set, then the default value is used.
**
** The numeric conversion is identical to strtoimax with the base inferred:
** - All digits up to the first non-digit characters are read
** - The longest consecutive prefix of digits is converted to a long
**
** Valid strings of digits are:
** - An optional sign character + or -
** - An optional prefix indicating the base (otherwise base 10 is assumed)
**   -- 0 prefix is octal
**   -- 0x / 0X prefix is hex
**
** Leading/trailing whitespace is ignored. Overflow/underflow will cause
** numeric conversion to fail.
**
** If no property with this key is set (or the key is NULL) or the numeric
** conversion fails, the default value is returned.
**/
int32_t property_get_int32(const char *key, int32_t default_value);/* property_set: returns 0 on success, < 0 on failure
*/
int property_set(const char *key, const char *value);int property_list(void (*propfn)(const char *key, const char *value, void *cookie), void *cookie);