SystemApi for dex module registration

PackageManager#registerDexModule() allows apps which can call
system apis to register a dex module with the Package Manager.

The PM may optimize the modules on the spot if needed. This is
particular useful for shared dex modules (e.g. chimera modules)
which are loaded in multiple processes.

Test:  adb  shell am instrument -e class
'android.content.pm.PackageManagerTests' -w
'com.android.frameworks.coretests/android.support.test.runner.AndroidJUnitRunner'
Bug: 37290820
Change-Id: I9ea8f937a76d2549a29e90a6c84c53c2e44a1ee4