Revert "Adds install source checks for accessibility service"

This reverts commit f1f2c847.
The stragery is chaged, we don't need to check the installer
nor if it is sytem app. We also delete the tests that are
obsolete.

Bug: 221318082
Test: AccessibilitySecurityPolicyTest, PolicyWarningUIControllerTest
Change-Id: Ide89213dbf90cdb57001e5b05a2eeefc35f6adee