ANDROID: db845c build: Use merge_configs

Previously the db845c build was cat'ng the db845c_gki.fragment
with the gki_defconfig. The correct way to merge fragments is to use
the merge_configs script.

Bug: 152231482
Test: Treehugger
Signed-off-by: Todd Kjos <tkjos@google.com>
Change-Id: I32d5d6576bb4bdc115b32e7e7ef3ef1e9daa2323