Disable emulator from platform builds.

Now that emulator prebuilts are available under prebuilts/android-emulator/,
disable building the emulator from source in all platform builds, except
if one defines BUILD_EMULATOR to 'true' in its environment.

NOTE: This patch should be applied after this one to avoid issues
      with the GPU emulation libraries:
            https://android-review.googlesource.com/93980

Change-Id: I53b2ada9ca0c2e159dccee7cdca7f55f6b0d1d42