AML: Make UserHandle.myUserId() available

Since the method UserHandle.myUserId() is a @SystemApi, build error
happens when building the mainline module.

This CL makes it available by building the APK with the system SDK.

Bug: 119749861
Test: mmm . (under frameworks/av/packages/MediaComponents)
Change-Id: Iedecf1a7c7de1279161601c1e88f959e89af73ac