Add native Android support.

[kinetiknz]

We can promote/demote native Android threads directly without involving JNI and android.os.Process#setThreadPriority by calling setpriority in bionic directly.

We need to get #31 landed to fix the CI runners, then this will require a small tweak for the Android CI build addition.

Closes issue #24.

[kinetiknz]

Tested with GeckoView in an Android emulator and confirmed to be working:

Before:

# ps -AT -w -o PID,TID,PRI,NICE,CMD | grep AudioIPC
   PID   TID PRI  NI CMD 
  2341  2504  19   0 AudioIPC Server
  2387  2749  19   0 AudioIPC Client

After:

# ps -AT -w -o PID,TID,PRI,NICE,CMD | grep AudioIPC
   PID   TID PRI  NI CMD 
  2658  2843  38 -19 AudioIPC Server
  2703  3202  38 -19 AudioIPC Client

Where a nice value of -19 is what we expect from requesting THREAD_PRIORITY_URGENT_AUDIO.

[padenot]

Very nice, thanks!