android: Add facility for java code to wait native code to be ready.
Especially when unzipping rockbox.zip, the native code can be initialized
a lot later than the java code. The java code needs to be prevented from
accessing rockbox structures (e.g. current_tick, event queues) before they're
ready.
This commit adds wait_rockbox_ready() and fixes dodgy behavior of starting
rockbox via widget play button, headset remote buttons or multimedia keys.
Also fixes wrong small list items before first redraw.
Change-Id: I1caf925e829a9c1c6bb6e0016d5c80574574c91e