Make TelecomManager#endCall a public API.

A broad category of apps such as wearable companion apps and call blocking
apps rely on the ability to reject a ringing call.
Previously this was achieved using a broken TelephonyManager API which
lacked permission checks.
To support these applications, removing the @hide attribute on the existing
TelecomManager#endCall API so that apps with the existing
ANSWER_PHONE_CALLS permission can reject ringing calls and end ongoing
calls.  Logically if an app has permission to answer a call, it should be
able to end it.

Test: Created test app to verify API permission checks.
Test: Added new CTS tests to cover this API.
Bug: 78290258
Change-Id: Ic6527969793ebe05eb9c5fa8205558ae788ea572