Insert dynamic dashboard tiles into optional placeholders

Insteads of inserting tile onto screen using absolute priority values,
now each page can have a placeholder preference, and at run time we will
add dynamic dashboard tiles to placeholder's place.

Bug: 32827787
Test: RunSettingsRoboTests
Change-Id: I1fe9e11dce4eb6fb4a9b56af05a2b8e5cdae00d2