Stop blocking preinstalled a11y tools from taking FLAG_SECURE screenshots.

Requires moving the caller of ScreenCapture's screenshot method from the
app process to the system_server process since capturing FLAG_SECURE
content, even for an app capturing its own content, requires permission
CAPTURE_BLACKOUT_CONTENT which is a signature permission.
system_server was already used for full-display accessibility screenshots.

Bug: 373705911
Test: atest AbstractAccessibilityServiceConnectionTest
Test: atest AccessibilityInteractionControllerTest
Test: atest AccessibilityTakeScreenshotTest
Flag: com.android.server.accessibility.allow_secure_screenshots
Change-Id: Ifb711222a42fa18abcabefafdef22daf46402955