Make test APIs synchronous

Post outgoing commands to providers to a handler rather than posting
incoming requests from providers to a handler in order to break deadlock
loops within providers. This makes it easier to the test APIs to behave
synchronously as a client would expect.

Bug: 136638281
Test: CTS
Change-Id: Id8ec2c18f287a31148cb854019ee52012db2a806
6 files changed