graphics: set maxThreads to 4 for the composer

There are implmentations that can dead lock when calling certain
functions in certain states.  It works fine before HIDL because
SurfaceFlinger calls those functions in another thread.  We allow
for 4 hwbinder threads in this commit to simulate how SurfaceFlinger
calls hwcomposer.

Bug: 38183197
Test: camera, videos, multi windows, screencap, screenrecord
Change-Id: Ie05b2ca349b8c1ed9a3ac962981434f2efee92d9