C2SoftGav1Dec: Add support for configuring threads using setprop

This helps in running some experiments to find decoder performance
for different number of threads.

Bug: 309029947
Test: adb root && adb shell setprop debug.c2.gav1.numthreads 4
Test: adb shell stagefright -s file.webm -N c2.android.av1.decoder

Change-Id: I7b4106981134355fe07ebc444d39fbac5afe41da