Set maximum attribute value to defined by spec Core 5.3

'The maximum length of an attribute value shall be 512 octets.' Core
5.3, Vol 3, Part F, 3.2.9

Test: atest net_test_stack_gatt
Sponsor: @jpawlowski
Bug: 150670922
Bug: 234988131
Merged-In: I214f2d1d142dc1ce5b24522bf820890d4599c702
Change-Id: I214f2d1d142dc1ce5b24522bf820890d4599c702

system/stack/include/gatt_api.h

diff --git a/system/stack/include/gatt_api.h b/system/stack/include/gatt_api.h
index 33a6eae..b38e943 100644
--- a/system/stack/include/gatt_api.h
+++ b/system/stack/include/gatt_api.h
@@ -247,7 +247,7 @@
 /* max legth of an attribute value
 */
 #ifndef GATT_MAX_ATTR_LEN
-#define GATT_MAX_ATTR_LEN 600
+#define GATT_MAX_ATTR_LEN 512
 #endif
 
 /* default GATT MTU size over LE link