Protect DisplayLeakActivity.setTheme() from receiving an incompatible theme

[ErikHellman]

No description provided.

[pyricau]

Can you add a comment to detail what's up, and also change to this:

  public void setTheme(int resId) {
    // insert here some comment explaining why we're doing this.
    if (resId != R.style.leak_canary_LeakCanary_Base) {
      return;
    }
    super.setTheme(resid);
  }

[pyricau]

I can't merge without you signing the CLA first :) https://github.com/square/leakcanary/blob/master/CONTRIBUTING.md

Also, it's a nit but it'd be nice to change this to return early, as I was showing above. Otherwise I'll just do a follow up commit, that's fine too.

[ErikHellman]

Signed CLA and made it return early. :)