ethernet: use short timeout for assumeNoInterfaceForTetheringAvailable

assumeNoInterfaceForTetheringAvailable checks if an interface is
available for tethering and throws and assumption failure accordingly.
If a physical interface is present, onAvailable() will be called almost
immediately, so there is no need to ever wait for TIMEOUT_MS. This is
especially important now that TIMEOUT_MS is being increased to 10s.

Test: atest EthernetManagerTest
Change-Id: I2f58d7690dbe4c25cfb01a8b29717cea6a01ab32