Fix gralloc direct channel bug and polish log messages

1) Remove obsoleted stub code so that second gralloc direct channel
   can be successfully registered.

2) Remove obsoleted misleading log messages.
3) Add missing log messages at failure to help debugging.

Bug: 36900495
Test: SensorDirectReportTest
Change-Id: I1d6941c4179338f6db8f84079844dce515b80516