Fix Bluetooth GATT API default handler assignment

Restores previous behaviour where GATT callbacks are invoked on the
binder thread and not the calling process main looper thread.

This fixes performance regressions as well as some
NetworkOnMainThreadException's for some applications.

Bug: 37544152
Bug: 37871717
Test: Covered by prior API tests.
Change-Id: Id8ab705dd4d7f00030e6ac29e056dde5180670e9
(cherry picked from commit 6bdc550e)