Use service dump for CarPowerTestService communication

In CarPowerHostTest, the main way CarPowerTestService communicated the
state of things on device, like listeners being cleared or set, was
through logcat. However, there was some flake observed with this method.
By utilizing the service dump method, the communication to
CarPowerHostTest can be more stable.

Flag: EXEMPT TEST_ONLY
Bug: 323561031
Test: atest CarPowerHostTest
Change-Id: I264fe1200fdf10b8c139bd4e15e05906e97d9b34