Performance improvements to call log scrolling.

- Remove call to CallUtil to check if video is enabled. It seems
like it's fine to include the content description of what the
call was, if it was a video call, even if there is not a
video-enabled call account.

- Factor out PhoneNumberDisplayHelper so it doesn't need to be
an instance. This reduces some extra calls to getDisplayNameHelper.
Probably a marginal difference, performance-wise, but it probably
helps a smidgen and also simplifies the need for creating and
passing or recalculating various instances of things.

TODO: It'd be much better if PhoneCallDetails had a builder.
It's terribly painful to fix all the tests when adding fields...

Change-Id: I6da13dc8b6b047043aba871796a8ed13b112a227