[Android WebAPK] Launch splash screen on top of web contents 2/4

This CL refactors WebappLauncherActivity in order to launch the
first run experience from WebappLauncherActivity instead of
WebApkActivity.

This change is desirable because:
- we want to launch the FRE prior to launching.
  OverlaySplashScreenActivity does not have any FRE-specific logic.
- This is in preparation for making SameTaskWebApkActivity singleTop.
  A singleTop activity cannot be relaunched on top itself (Intent#onNewIntent()
  is called instead.)

BUG=918459

Change-Id: Idd04f5534f7abc6a561a20273daa70d9b970d109
Reviewed-on: https://chromium-review.googlesource.com/c/1394936
Commit-Queue: Peter Kotwicz <pkotwicz@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#620919}
7 files changed