Fix non-Android build targets

Non-Android build targets are missing sys/prctl.h header and functionality
should be disabled for them like it was done previously inside
sched_policy.cpp. Also make the set_sched_policy/get_sched_policy
functionality backward compatible by creating stubs for non-Android
targets.

Bug: 111307099
Test: built sdk_gphone_x86-sdk_addon_mac target using forrest

Change-Id: I1c195267e287a84a21c588bd61d7c452bff6cfbe
Merged-In: I1c195267e287a84a21c588bd61d7c452bff6cfbe
Signed-off-by: Suren Baghdasaryan <surenb@google.com>