Use FrameLayout for preferences container

With a LinearLayout, the preferences view space is cutting off the space
to show the "No unused apps" text. A LinearLayout seems to have been
used due to a legacy UI (see ag/6179722, ag/14803755) but doesn't really
make sense for the current UI or a generic case.

Bug: 193696585
Test: manual
Change-Id: I6db54a0fb1ad900f08a817fcb0edef75ccb23ebd
2 files changed