Update LauncherActivity to work well in "Single Activity TWA" mode

The changes are:
1) LauncherActivity should reuse session id in order to route new View
intents to existing TWA.
2) Intent flags are not needed, as they are added in http://crrev.com/c/1349653
(and without that CL they don't work anyway).
3) LauncherActivity closes when user returns back to it after closing a
TWA.
4) Service connection is kept alive until onDestroy instead of onStop,
so that clients can keep communicating with TWA while in background,
should they write an extended LauncherActivity.
5) Updated javadocs.

Companion CL: http://crrev.com/c/1349653

Bug: 907796
Change-Id: Iadd28a407129757da85932722d691d777e9868fe
Reviewed-on: https://chromium-review.googlesource.com/c/1349363
Reviewed-by: Benoit L <lizeb@chromium.org>
Reviewed-by: Peter Conn <peconn@chromium.org>
Reviewed-by: Yusuf Ozuysal <yusufo@chromium.org>
Commit-Queue: Pavel Shmakov <pshmakov@chromium.org>