Use only one layout, with a dynamically sized PagerAdapter

it is more straightforward to only have one viewpager that can be
flipped through. This makes it easier to restore the user's default way
of connecting (bluetooth via wifi) and also simplifies the setup and
the what-tab-am-I-on checks.

* Remeber what tab (wifi/bluetooth) the user last used and restore that
  on next launch
* respect Android's guidelines and ask the user whether Bluetooth should
  be enabled when the user switches to the BT tab and BT is disabled.
  → if the user declines, select wifi tab instead

Fix a lifecycle problem (bt connection would be cut if bt was not
enabled before launching the remote, only restore disbled state if
really finishing, not on configuration change like rotating the screen)

Change-Id: Ice3a5c877a2a4810a80a0f76edea713700fe9c8c
Reviewed-on: https://gerrit.libreoffice.org/7090
Reviewed-by: Andrzej Hunt <andrzej.hunt@collabora.com>
Tested-by: Andrzej Hunt <andrzej.hunt@collabora.com>