zzz and sponge pointed out that the router needs to be managed by whatever
Activity happens to be active (in case an automatic start is required). This
commit reverses the initial migration of I2PActivityBase to a Fragment (code is
broken now).

The methods in TunnelConfig and TunnelUtil are taken almost verbatim from
IndexBean (modified to be static in TunnelUtil). The tunnel creation wizard
works now, and the new tunnel will start if auto start is enabled.

This is better for smaller devices. Larger devices will be customized later.

Most of the getters in TunnelEntry are almost verbatim from
net.i2p.i2ptunnel.web.IndexBean and net.i2p.i2ptunnel.web.EditBean

There are a lot of changes here:
- The main_content FrameLayout is now main_fragment (more logical name).
- TunnelEntry now stores a Context instead of a TunnelEntryLoader (so
  it can be instantiated inside the detail Fragment).
- The activity_navdrawer layout is now an alias to a one-pane or two-pane
  layout depending on screen size.
- Subclasses of I2PActivityBase can now override canUseTwoPanes() to enable
  and use the two-pane layout (for devices that support it).