ANDROID: sched: Fix wake_q length tracking

The current approach to carry the wake_q length is exposed to an
intertask stack access. For example, if A sets the wake_q_head for
B but is preempted before it is able to set it back to NULL,
then B continues to point to an address corresponding to A's stack.
If B is then woken up by another task, it ends up accessing
the address pointing to A's stack. This causes a memory fault.

Replace this with a simple parameter which indicates the number
of tasks that are being woken up as part of the same event. This
avoids saving and accessing on stack pointers.

Bug: 173981591
Change-Id: I0031747d79a27673e680f7b1121eb4896ac7c699
Signed-off-by: Shaleen Agrawal <shalagra@codeaurora.org>