Announce private profile screenshot saving.

Extract announcement logic behind a flag (not much to extract, but it's
one less thing in the controller and now can be tested).

Test: atest com.android.systemui.screenshot
Flag: com.android.systemui.screenshot_private_profile_accessibility_announcement
Bug: 326941376
Change-Id: Iec464e4487e14a5a44298521e18d5b895eb27e90