[MTE] Add SYNC compat feature & disable for system apps.

Adds the synchronous compat feature for MTE. It takes precedence over
the ASYNC feature if both are specified.

Also disable MTE by default for system apps. MTE is entirely opt-in, not
opt-out.

Bug: 135772972
Bug: 177438394
Test: Boot FVP.
Change-Id: Id4c356912163b646abef82b9cce74e9cdbcf1d3d