Prevent rare freeze on Android 4.2 through 4.4

Prevent rare freeze on Android 4.2 through 4.4

* src/android.c (android_run_select_thread, android_init_events)
(android_select): Enable self-pipes on all Android versions <= 21.
The Android C library provides a functioning pselect on these
systems, but it does not apply the signal mask atomically.
(android_run_select_thread): Correct typo.  This never produced
any adverse consequences, as the relevant signals would already
have been blocked by `setupSystemThread'.

Do not merge to master.