Fix Android thread priority

[chucknology]

Android threads are Linux threads which differ from the POSIX standard.
Calling pthread_setschedparam does not produce the intended effect on Android.

[paulfd]

@atsushieno you have more Android experience than I do, if you find some time to have a look I will defer to your judgment :) Thanks!

[atsushieno]

@paulfd okay, but I was not aware of this platform-specific difference either, so I'm reviewing from that stand point.

@chucknology Reviewing without actually running code so far - I agree to the change in general, but it seems that the intent of setting priority there is so that it gives "moderately higher" priority than normal thread, not "highest" (-20). The default value for config::backgroundLoaderPthreadPriority used in non-Android code is 50, so I would say -10 is better (if config is not considered).

IfUnless there is still reason to set -20 (or it does not make sense to use values like -10), I would suggest @paulfd to make another commit to make such one-liner change and merge it.