Fully support Lisp threads on Android

Fully support Lisp threads on Android

* src/android.c (struct android_thread_event_queue): New
structure.  Move global pselect arguments, mutexes, and
semaphores, and pipes here.
(struct android_event_queue) <select_mutex, select_thread>:
Remove to the above-named struct.
(android_init_thread_events, android_finalize_thread_events)
(android_get_poll_thread): New functions.
(android_run_select_thread): Accept a set of mutexes and
thread-local data as the first argument, and operate with them
rather than globals.
(android_handle_sigusr1): Rename to
`android_handle_poll_signal'.  Set thread-specific cancellation
indicator.
(android_init_events): Properly abort after a fatal condition.
Enable interruptible polling on Android 5.1 and later, not 4.2.
(android_select): Never check for queries here, but in
thread_select, if threads are enabled.  Adapt to per-thread
polling threads and only enable interruptible polling on Android
5.1 and later.
(android_before_select): New function.

* src/android.h (android_before_select): New declaration.

* src/thread.c (thread_select): Call `android_before_select'
before the global lock is released.

(cherry picked from commit 8df582a46836e312ef3bfc36c8038b5f4a2c0d9b)