Do not use hidden API UserManager#getPrimaryUser

Use UserHandle#isSystem instead. They're not the same when
ro.fw.system_user_split is set, but for VT functionality
this change is benign.

Bug: 137202333
Test: make
Merged-In: I1a59f78810d073d1782d8ad8498a36ea85428c09
Change-Id: I1a59f78810d073d1782d8ad8498a36ea85428c09
1 file changed