Minor optimize for Text (Add API for length of text + Cache DPI)

Minor optimize for Text (Add API for length of text + Cache DPI)

Since we don't need whole text information at UpdateRenderer,
just get length of text.
(Actually, we only need whether text is empty or not)

+

When we use PIXEL_SIZE instead of POINT_SIZE, FontClient::Get() give some overhead.
To avoid singletone service access, get DPI only one time, and cache it.

TODO : Should we consider DPI changed case?

Change-Id: If1e5b29b8c0b243753c19be20fbef432e6f62f15
Signed-off-by: Eunki, Hong <eunkiki.hong@samsung.com>