Switch all screenshot saving paths to ImageExporter

Step toward fully deprecating SaveImageInBackgroundTask. Also removes
shouldShowUiOnExternalDisplay boolean (unused).

Bug: 352308052
Flag: com.android.systemui.screenshot_save_image_exporter
Test: atest, manual test of toast pathway with flag on/off
Change-Id: I038508354c1aeb8a8096b4230108baf194f4381e