Clamp brightness before setting it

- Screen brightness requests of "0" (int) = OFF, this is clamped to
  PowerManager.BRIGHTNESS_MIN and PowerManager.BRIGHTNESS_MAX

Test: manual, atest DisplayServiceTests
Flag: EXEMPT trivial bug fix
Bug: 364835795

Change-Id: Ic21ea10108122925a85adf7cac1afc85c68334e0