Defines Gamepad-specific keys and axis in UinputBridge.

Adds the ability to open gamepads in the JNI layer of server.tv.
Major changes:
  - defined gamepad specific keys and axes
  - centralized /dev/uinput file descriptor management during the
    open method (since gamepad and remote have similar open logic)
  - added gamepad open, keysend and axis send methods.

Test: Android remote service still works on a clean build & flash.
     (tested remote service on my phone and UI navigation).
Test: as part of a larger chage chain, opened gamepad devices
      and observed /dev/input/event* behaviour for axes and keys.
Bug: 150775713
Bug: 150776008
Bug: 150784056
Change-Id: I2eee6844dd85346753ec62430c234cc7e0179d3b