Ensure non-empty selected routes in session

This is a partial revert of
I0c12d2b63b0c3969c91e0082f7821e342360b724, which allows a race
condition where the available routes don't include the selected
route, because both collections are obtained without holding the
lock that guards both collections.

While this change doesn't ensure the transferable and selectable
routes of the session are consistent, it does prevent an exception
due to empty selectable routes.

Bug: b/416911854
Test: atest CtsMediaRouterTestCases
Test: Manual using a BT device and a wired headset.
Flag: EXEMPT BUGFIX
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:d1b3245c261cabcc936af263e3f71b79c9aeecc5
Merged-In: I6a022c6098daf8282e6085bb671173f574e63a4f
Change-Id: I6a022c6098daf8282e6085bb671173f574e63a4f