Do not construct std::function with Allocator argument

Bug: http://b/116702964

This breaks when built with C++17, where these constructors are removed.
Looks like libc++ ignored the allocator arguments anyway - so this
should be NFC even with C++17.

Test: memunreachable_test on the host and Sailfish with C++14, C++17
Change-Id: I638326a900781f64d0562b5ec38e990659967039