We switch from cmake to ndk-build to make it easier for builders
in not requiring an additional tool installed. The JNI build is
so simple so we don't really need much of a build tool anyway.
Previously we waited for all opened file descriptors to the terminal
to be closed. This caused problem when e.g. running "sleep 900 &"
and then exiting the shell, with sleep keeping the session alive and
had to be killed manually (killing the process group did not help -
the shell had already exited and was in zombie state). This is also
what most other terminal emulators do.
Relatedly, switch to sending SIGKILL to force quit a session instead
of SIGHUP, since SIGHUP can be ignored.
This fixes https://github.com/termux/termux-widget/issues/2, which
was caused by the terminal launching the terminal session process
before the terminal size was known.
Also remove the built JNI libraries from source control.
