Start platform compat service eariler.

It's needed by ActivityManager and PackageManager.
Also use a constant in Context for the name.

Test: flashed device with ag/9025572 and ag/9204795 and the platfrom
compat was accessible.
Bug: 137769727

Change-Id: Ie1130a3f0bdd1769fe0755db0089702ea64d9db6
Merged-In: Ie1130a3f0bdd1769fe0755db0089702ea64d9db6