Retry the call that fetches the tethering offload HAL.

The CL that moved the initialization of the tethering offload
config HAL from C++ to Java caused the code not to retry
fetching the service if it is not ready when tethering is
started. This is because the C++ version of getService() retries,
but the Java version only retries if getService(true) is called.

Make the new code retry as well.

b/152430668#comment4 asserts that the fetch will be retried only
if the service is installed on the device, so the retries should
be attempted (and thus should not have any startup time impact)
on devices that do not support tethering offload.

Bug: 152430668
Test: builds, boots, tethering offload works
Change-Id: I093f127d90b2aa1b13eb0748378a24726d419472