Fix shim layer bugs for Sensors HIDL wrapper.

i) Increase the number of default threads to 2.
ii) Handle the flush complete event correctly.

Bug: 35189074
Bug: 35067266

Test: Test that sensors work in both binderized and passthrough modes.
      Ensure that camera photosphere works.

Change-Id: Ic8dadf03395b4e96092ae962cf790c93d65a1c8d