ANDROID: sched: add "frozen" field to task_struct

use one of the ANDROID_KABI_RESERVED fields for the v2 freezer "frozen"
bit.

Bug: 163547360
Test: built and booted
Signed-off-by: Marco Ballesio <balejs@google.com>
Change-Id: I7d7aed173a09580b8eff1ccf39ca4f162fbaddc8