Fix flaky tests in ThemeBinderServiceTests

The tests were failing due to a user ID mismatch. ThemeBinderService uses the calling user's ID, but the tests were hardcoded to use user ID 0. This caused theme settings to be written for one user and read for another, resulting in test failures.

Fixes: 433274796
Flag: EXEMPT test only
Test: ThemeBinderServiceTests
Change-Id: I54ce2caef46da3f4e6d138b1cdaa5fd6dfc1491e