Improve the logic of checking device idle

* #checkDeviceIdle encapsulates the logic of checking device idle,
  making the life of the caller easier.
* Now timeout-check always happens after device-idle-check.
  No clamping is needed w.r.t PENDING_CHECK_MILLIS.

Bug: 235306967
Test: atest CtsStagedInstallHostTestCases
Change-Id: Ibbdd34518ce9f53b5053eff18826e2dd72edac6b