Camera: Cache tag id to avoid repeatative lookup

Looking up tag id based on tag name for each key is potentially time
consuming. Instead, cache the id in the Key object so that only when the
first getTag() triggers a look-up. All subsequent calls doesn't do
look-up.

Bug: 144028609
Test: Run GCA and observe CameraMetadata CPU consumption
Change-Id: Iadd92e7e3b6cb4d610a6c9704ca2066291291f43