adb: make adb_thread_func_t return void, add adb_thread_exit.

Windows restricts the return value of threads to 32-bits, even on 64-bit
platforms. Since we don't actually return meaningful values from thread,
resolve this inconsistency with POSIX by making adb's thread abstraction
only take void functions.

Change-Id: I5c23b4432314f13bf16d606fd5e6b6b7b6ef98b5