Add a config resource to disable key-chord screenshotting

The key chord screenshot mechanism introduces significant latency into
processing of volume-key input; enough to be quite noticeable and
annoying on some kinds of device.  This patch introduces a new config
resource entry ("config_enableScreenshotChord"), true by default, so
that products on which this functionality is inapplicable can avoid
its runtime overhead.

Bug 6039047

Change-Id: I968ddf9046741da35988310b7893fae2c0369beb