Add a system prop to control shader cache priming

Shader cache priming in surface flinger can take a significant amount of
time at system start, during which the device is basically stuck. Add a
system property to allow devices to disable shader cache priming if they
want.

Bug: 136204746

Test: Added logs locally to confirm setting the prop works as expected:
0 = don't do shader cache priming; any other value = do shader cache
priming. Default value is 1.

Change-Id: Iafd1823bcb6a96d69ca0bc7a4c0b847f8ceb9e77