Intent.java EXTRA_USER clarifications

In Intent.java:
* Clarifies userId vs. UserHandle vs. uid.
* Fixes javadoc links to EXTRA_USER_HANDLE and UserHandle
* Updates javadoc to mention that EXTRA_USER is available
for some SystemApis.

Bug: 189409989
Test: Compiles (just comments)
Change-Id: I63496ddb5ed4ddc7bc26540c6dad7cbe7911520d