Annotate TextServicesManager with @UserHandleAware

This is a follow up CL to my CL [1], enabled TextServicesManager to
determine the target user ID by Context#getUser().

This CL aims to clarify what methods in TextServicesManager follow
this semantics.

Note that @UserHandleAware is still a kind of hint for readers.  There
should be no change at the runtime.

 [1]: I31dda3ae8795190d44b0622b8335c34ddbc5dd48
      9f141ee8

Bug: 192412909
Test: presubmit
Change-Id: I00bd99ef9d83a13bd60a5aefd22094725b67e271