leaudio: Reserve GATT handles for GMSC and GTBS

With this patch, GMCS and GTBS will always have same handles. This is to
make sure, that peer does not have to rediscover Android database
because of those services

Bug: 150670922
Tag: #feature
Sponsor: jpawlowski@
Test: Compile
Change-Id: I4b9daee91f45ffb95752f117f7af138fda488696