adb: wait for devices to come up instead of sleeping for 3s.

Replace a hard-coded 3 second sleep with logic to wait until we've
scanned USB devices once and they've all come online.

Before:
    adb shell true  0.00s user 0.00s system 0% cpu 3.047 total

After:
    adb shell true  0.00s user 0.00s system 9% cpu 0.041 total

Bug: http://b/37869663
Test: `time adb shell true` after adb kill-server
Change-Id: I251d42afb885908ed9d03167287594ea16650d3f