qs: fix network tile description

Fix period at the end of the network mobile description, ensure that there isn't
trailing whitespaces and only call one time to the cleaner method when
the description change. Clean up unused imports.

Change-Id: I9b965015e30e44efa00220e91da7e8b3e773a0f1
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>

Conflicts:

	packages/SystemUI/src/com/android/systemui/quicksettings/MobileNetworkTile.java