leaudio: Make sure all unit tests depeds on flags are tested

This patch removes all the TEST_F_WITH_FLAG test cases and adds
fake_com_android_bluetooth_flag.cc.

Fake flag_provider differs from the original one only with a cache
functionality, which is removed from a fake one. This helps to control
flag state for each test cases, without the risk that flag state is
cached from the previous test case.

Bug: 331775328
Flag: TEST_ONLY
Test: atest bluetooth_le_audio_test bluetooth_le_audio_client_test
bluetooth_test_broadcaster
Ignore-AOSP-First: build error due to security fix not being in aosp

Change-Id: Ibabd93937c5e2ccf48fda19e229f02320242d95d