Suppress Splash Screen on widget reconfiguration activity start

When a widget configuration activity is started, it's started from
the launcher. Under some circumstances the launcher's splash screen
would be shown, rather than the splash screen corresponding to the
tapped app.

To avoid confusion, we suppress the splash screen for configuration
starts.

Bug: 190153065
Test: manual
Change-Id: Ib18851f1bb50b64bc5d47b8aa5931490c969b54b