[Hotword] Move disable test model away from teardown

If we disable the test model in the tearDown method, it may have
timing issue. The test VoiceinteractionService may not be the active
VoiceinteractionService at this time. It causes the unexpected error
in tearDown. Move the disable logic into needed test cases.

Test: atest HotwordDetectionServiceBasicTest
Test: atest --test-mapping
      ./cts/tests/tests/voiceinteraction/TEST_MAPPING:presubmit-large
Bug: 278384069
Change-Id: Iea088ebfd0f047b5d4a7598cc95033de2821469a