Do not process messages until new messages are feeding

There may be race condition as the initial messages may have been
processed before the TestableLooper is initialized.

Bug: 176119417
Test: atest TeleServiceTests:com.android.phone.RcsProvisioningMonitorTest
Change-Id: I7dead5b72fb465e7c32964e6f0a42b1efb1a7765
1 file changed