ANDROID: mpam_policy: Make the tasks file writable

This means tasks can now be assigned to a non-default partition, which
requires syncing the CPU partition value (MPAM0_EL1.PARTID_D) with the next
task at context_switch time.

trace_android_vh_is_fpsimd_save() is just in the right place for this
operation, so that's what is being used.

Change-Id: Id6196c446311df466f5fda5863a820375ae56e10
Signed-off-by: Valentin Schneider <Valentin.Schneider@arm.com>