Remove isAppSourceCertificateTrusted implementation

... since it's already returning false constantly.

The removal includes:
* the manager code that calls the service
* the service implementation
* the shell tool for key chain manipulation

The next step is actually mark the API as @removed. As the API is being
finalized, this will be done after the new API level is introduced.

Ignore-AOSP-First: Some of the previous cleanup is only done internally.
                   The code should be made public soon anyway.
Flag: EXEMPT removing deprecate_fsv_sig
Test: atest FileIntegrityManagerTest
Bug: 277916185
Change-Id: I23ee7d569333bee243416c463e4391a34e730e00