Add Transit Center View for user-created stop groups

[diveshpatil9104]

Closes #1221

Problem

Users who transfer at transit hubs (e.g., Bellevue Transit Center) have to star individual stops and scroll through a flat list to compare arrival times. There is no way to group related stops together.

Solution

Adds a "Transit centers" feature that lets users create named groups of stops, accessible from the navigation drawer. Users can:

• Create, rename, and delete transit center groups
• Add starred stops to a group via a multi-choice picker
• View all stops in a group with a single tap
• Toggle between "all stops" and "starred only" within a group
• Remove individual stops via long-press context menu

Tapping a stop opens the existing ArrivalsListActivity — no new arrivals UI needed.

Demo

center.mp4

Implementation

Database (v33 → v34):

• transit_centers table — stores group name + region ID
• transit_center_stops join table — maps groups to stop IDs
• Cascading ++oldVersion migration so all upgrade paths work

UI:

• TransitCentersFragment — CursorLoader + ContentObserver list of groups
• TransitCenterDetailFragment — two-step query (get stop IDs, then query Stops with IN clause)
• AddStopToTransitCenterDialog — multi-choice picker for starred stops
• TransitCenterDetailActivity — hosts detail fragment, uses UIUtils.setupActionBar() + android.R.id.content pattern

Navigation:

• New NAVDRAWER_ITEM_TRANSIT_CENTERS in drawer (between Starred Routes and My Reminders)
• Show/hide fragment pattern in HomeActivity matching existing nav items

New files (7)

File	Purpose
TransitCentersFragment.java	List of transit center groups
TransitCenterDetailActivity.java	Hosts detail fragment
TransitCenterDetailFragment.java	Stops within a group
AddStopToTransitCenterDialog.java	Starred stop picker
transit_center_list_item.xml	Group list item layout
transit_centers_options.xml	"Add" menu
transit_center_detail_options.xml	"Add stops" + "Toggle starred" menu

Modified files (6)

File	Change
ObaContract.java	TransitCenters + TransitCenterStops classes
ObaProvider.java	DB v34 migration, URI matching, CRUD
NavigationDrawerFragment.java	New nav drawer item
HomeActivity.java	Show/hide transit centers fragment
AndroidManifest.xml	Register detail activity
strings.xml	14 new string resources (including plurals)

Test plan

• Open nav drawer → "Transit centers" → empty state shows "No transit centers"
• Tap add → enter name → group appears in list with "0 stops"
• Tap group → "Add stops" → pick starred stops → stops appear in detail view
• Toggle "Starred only" → filters correctly
• Tap a stop → ArrivalsListActivity opens
• Long-press stop → "Remove from transit center" → stop removed
• Long-press group in list → rename works, delete works
• Switch regions → groups from other regions not shown