[1/2] SystemUI: Update network traffic indicator

Android measures the amount of data transferred in bytes,
however, by convention, data rates should be measured in bits
per second.

*Convert the change in total data and time to bits and seconds,
and then divide the amount transferred by the number of
seconds that have elapsed.

*Update the labels and have them use the correct SI prefixes.

*Factor out or add @hide to public methods and constructors

*Remove all Runnables and Messages from Handler queue when
disabled

*Minor formatting changes to stay consistent with Android's
code-style guidelines.
https://source.android.com/source/code-style.html

PS2: Set maximum digits to prevent it from filling the statusbar
PS3: Rebase
PS4: Add ability to display either bits or bytes

Change-Id: I9add610a8d14f5a48f2af0a82ced0c186224e282