Wait for restore thread to start before checking for confirm button

Attempt to fix b/36363386.  It's possible that we may timeout waiting
for the confirm button before the adb restore command has even been sent.

Test: cts-tradefed run cts -m CtsBackupHostTestCases
Bug: 36363386
Change-Id: I9eeb0b6ae79fdd7aa3790448789f95c8029d152e
1 file changed