Add @VisibleForTesting in PbapStateMachine

Unlike atest results, Mockito cannot mock package-private classes.
This CL makes the PbapStateMachine as public when testing so that
the test does not fail.

Bug: 237548430
Test: atest BluetoothPbapAuthenticatorTest
Ignore-AOSP-First: The CL that adds BluetoothPbapAuthenticatorTest
    in AOSP already has this change.
Change-Id: I61c2692e9b00496883d0adc203a2edd3445922bc