Fix POSIX timer thread naming.

Spencer Low points out that we never actually set a name because the constant
part of the string was longer than the kernel's maximum, and the kernel
rejects long names rather than truncate.

Shorten the fixed part of the string while still keeping it meaningful. 9999
POSIX timers should be enough for any process...

Bug: https://code.google.com/p/android/issues/detail?id=170089
Change-Id: Ic05f07584c1eac160743519091a540ebbf8d7eb1