Create ContextualOnceCallback, to bind together a context and a callback

This way, we don't need to manually pass handlers around.

Will eventually subsume and become OnceCallback for simplicity, but keeping
ContextualOnceCallback as the name for now during migration.

Test: cert/run --host
Test: fuzz/run --host bluetooth_gd_hci_layer_fuzz_test
Change-Id: I4f686d134773877514b7f41ca97337001bdeb971