Remove CLASS_EXISTENCE_CHECK metadata from ConnectivityCallListenerService

Addressing security vulnerability b/501155244 by removing the
'android.telecom.CLASS_EXISTENCE_CHECK' metadata. The corresponding
logic in Telecom that checked this metadata has been removed as it
posed a risk of arbitrary code execution in system_server.

Bug: 501155244
Test: atest InCallControllerTests, Manual
Flag: EXEMPT BUGFIX
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:caa9d5b8040a75b083fabd7fbe44d0e1645ba1c8
Merged-In: I09d72cbdef00007e21f023731b45bae25cc0a7f9
Change-Id: I09d72cbdef00007e21f023731b45bae25cc0a7f9