Screenshots: use ImageExporter

Use a single tested module for image export steps which is
now shared between the standard flow and the long screenshot
feature.

Test: ImageExporterTest android.assist.cts.ScreenshotTest
      ScreenshotNotificationSmartActionsTest
Bug: 175744393
Fix: 175744393
Change-Id: Id08716736edb4ac1d00c13224e7763e8ca53c979