Fix DynamicSystemClient.start() exceptions

* Remove calls to featureFlagEnabled() as they are causing avc denials,
  plus the feature flag is already deprecated.
* Add Intent flag FLAG_ACTIVITY_NEW_TASK to fix the
  AndroidRuntimeException, as the Context object used to startActivity()
  may not be an Activity object instance.
* Check the nullness of mListener before using it to fix the
  NullPointerException.

Bug: 176795908
Test: atest \
  cts/tests/tests/os/src/android/os/image/cts/DynamicSystemClientTest.java
Change-Id: I265d9912e76c39406e0adeacd2490b62873c94e4