Only current user can update GrammaticalInflection for MUMD

Android auto supports MUMD user model where background users have
UI access on assigned displays, a.k.a. visible background users.
In Automotive's multi-display where passengers
(modeled as visible background users) can interact with the display
in front of them concurrently with the driver (modeled as the the
current user) who interacts with driver's display.

The change for this CL will apply on the devices who enables
config_multiuserVisibleBackgroundUsers. Currently only android auto has this
enabled.

Android auto also doesn't support profiles so the change is only for current
user. After this change, on Andorid Auto's MUMD config, background visible users
can't change the GrammaticalInflection, only current user can change the
GrammaticalInflection. The change has no effect on other form factors.

Flag: EXEMPT minor fix

Bug: 352784696
Test: atest CtsGrammaticalInflectionTestCases
Change-Id: I6735e7b309971866aca406b1e2035a225df35bdf