adb-remount-test: wait-for-screen

If we reboot too agressively after a fresh flash either just before
test is run, or after vendor is flashed, we run the risk the device
will consider it a bad boot and head towards recovery or revert to
previous system.

Add checks to wait for the screen.

This can result in the test reporting issues with boot complete,
which will not fail the test currently, but can be used to determine
if the device under test is in a boot loop or fragile state.

Test: fastboot flashall ; adb-remount-test.sh
Bug: 132070014
Change-Id: Ia1b3800c44222cb8fbd9b00e897b32a256996ebc