GATT discovery refactor

Currently, when doing service discovery, we store pending discovery
content into tBTA_GATTC_ATTR_REC structure, and rewrite it into cache,
service after service during discovery.
From now on, we save discovery result directly into pending_discovery,
and at end of discovery we swap it with cache. This remove unncecessary
intermidiate step, and simplify the discovery.

Bug: 67057055
Test: GattReadTest
Change-Id: Id09026b6fb6df04eaf0a0ebefeb6e35b3d175f62