Add an override for getUser in ContextWrapper

This is to fix an issue when calling getUser for a context that
is different than the context of the current foreground user.
getUser for a ContextWrapper would call the base
class Context#getUser which returns the user of the current foreground
process rather than returning the user of the calling context.

Currently, this is a bug that does not meet the ContextWrapper class Javadoc.
It looks like the ContextWrapper call was missed in Ib772ec4438e57a2ad4950821b9432f9842998451.

Fix: 144024489
Test: called the API from a test App with a different context than the
current application context.

Change-Id: I2d24f5141e8cbc2546c8dc5894a1aeab376b7632