Don't fork USAPs with open argument buffer

Delay execution of any zygote command that may end up forking USAPs
until we've released our own ZygoteCommandBuffer. Otherwise we fork
the child with an open ZygoteCommandBuffer, preventing it from
opening its own.

Bug: 182084594
Test: Build and boot AOSP, with USAPs enabled and disabled.
Change-Id: I2366367d2155a120092fbdcae13cbb3d7f4e1d16