Update input method menu to use proper context.

Was not using the system ui context so it wasn't getting
themed properly.

Bug: 36059431
Fixes: 37788566
Test: built sailfish-userdebug, opened the keyboard by selecting an
    EditText, long-pressed keyboard to switch keyboards, observed
Change-Id: I6ec2381620d1a88c025a0f9228bc58bc11db6bc4