Use pExitEncodeEvent instead of thread cancellation on unix as well

This works now that we've got a suitably working implementation
of WelsMultipleEventsWaitSingleBlocking.
Remove the now unused thread cancellation support

This makes the thread library build on android - android does
not have pthread_cancel.
Enable multithreading on android
Use WelsThreadJoin on windows as well

This avoids using a separate event just for signalling that
a thread has finished running.
Merge WelsThreadDestroy into WelsThreadJoin

Now calling WelsThreadJoin is enough to finish and clean up
the thread on all platforms.

This unifies the thread cleanup code between windows and unix.

Now all of the threading code should use the exact same codepaths
between windows and unix.
Merge pull request #442 from mstorsjo/unify-thread-cleanup

Unify the thread cleanup code, enable threading on android