adb: split up adb_auth.cpp.

All of the functions in adb_auth.cpp were used in only one of
adb/adbd. Split up them up into adb_auth_host.cpp and adbd_auth.cpp
respectively.

Bug: http://b/29273531
Test: built and flashed bullhead, adb still works
Change-Id: Ib610c5157522634cc273511175152f1306cc52a7