Change max attribute length from 600 bytes to 512 bytes

This change removes BTGATT_MAX_ATTR_LEN, defined in bt_gatt_client.h as
600. It is causing problems and differs from the correct max attribute
length in Android, which is 512 bytes. A new correct max attribute
length is defined in bt_gatt_types.h and used instead. In a follow up
CL, another GATT_MAX_ATTR_LEN defined in gatt_api.h will be replaced
with the new one added in this CL.

Bug: 301025186
Test: m com.android.btservices
Change-Id: I0779f63202163fbae282dcd50ce76b53611cfeda