android/skin/keyboard: Fix SDL2 keyboard handling.

This modifies the way kEventTextInput events are handled by
the SkinKeyboar class:

- Before:

    * Each kEventTextInput event generated a corresponding
      a single up/down sequence of keycodes.

    * SDL 1.x generated two kEventTextInput events per
      keypress, one for down, then one for up.

    * SDL 2.x only generates a single kEventTextInput
      event (down), by design.

- After:

    * Each kEventTextInput event generates two sequences
      of keycodes, one for down, followed by one for up.

    * The SDL 1.x backend is modified to only generated
      a single kEventTextInput on SDL_KEYDOWN events only.

    * The SDL 2.x backend is unmodified.

Change-Id: I7a4deca549720eb5898e7c576917913198cd817d
Merge "android/skin/keyboard: Fix SDL2 keyboard handling."