BandwidthController: fix format string bugs

In 87666694, a new %s
was added to ALERT_IPT_TEMPLATE. Not all users of this string
were updated.

This change modifies ALERT_IPT_TEMPLATE to be a #define, which
allows gcc's format string detection work.

Add -Werror=format to error out on any string format warning.

Testing: Code compiles. I don't know how to test this change
properly.

Bug: 5948299
Change-Id: I0ec307972e6bf50abd8ba099166c22069a6c6580