De-contextify volume/Events MetricsLogger calls.

A small cleanup - replace calls to MetricsLogger's deprecated static
methods with calls to the member functions of the logger instance that
the code was already constructing.  This makes the Context argument of
Events.writeEvent() unnecessary, so removed that as well.

Test: atest SystemUITests
Test: mp droid + manually verify MetricsLogger logging.

Change-Id: I8ca380ca56f51c414c17858dfd5b932ba02d4df0