bionic: make NONBLOCK call to getrandom

By default getrandom() blocks if the entropy pool has not yet been initialized.
This will be an issue when init was first executed in some kernels.

This CL makes a check of getrandom readyness, by adding the GRND_NONBLOCK flag.
In such case, getrandom() does not block returns -1 with errno set to EAGAIN.

Test: on M/S devices
Bug: 33059407
Change-Id: I2a2ba8372a5e1c336852ba2ab77cdaac03c90389