AudioPolicy: add test flagging for focus policy tests

(Note there is no @TestApi for @SystemApi, so rely
  on javadoc to document that the new method is
  for testing purposes only)
Add method to flag a focus policy as a test policy for
  devices that run their own focus policy to customize
  audio focus management. When a test policy is installed
  it will replace the current one, and will restore it
  when it is unregistered.
Add nullability annotations in Builder.

Bug: 123717363
Test: make system-api-stubs-docs-update-current-api
Test: gts-tradefed run gts --skip-device-info --skip-system-status-check com.android.compatibility.common.tradefed.targetprep.NetworkConnectivityChecker -m GtsGmscoreHostTestCases -t 'com.google.android.gts.audiopolicy.AudioPolicyHostTest#testFocusPolicy'
Change-Id: I572487bb53f85aade22da8529db9491a542ef0db