HdmiCecControl.keydown: Reduce interval to 300ms, and calculate deadline
HdmiCecControl.keydown: Reduce interval to 300ms, and calculate deadline

...instead of sleeping for 300ms. This way the keypresses will arrive
300ms apart, instead of 300ms plus however long the call to the previous
`libcec.Transmit` took.

I assume that `self.lock.wait(0)` (if `libcec.Transmit` took longer than
300ms) will release the lock if other callers (most importantly
`keyup`!) are trying to acquire the lock.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester