Fix Display SurfaceControl leak

The DisplayContent was merging its transaction into the
RootWindowContainer's pendingTransaction. However, that transaction is
never applied so it was stuck in that transaction forever.

There's no need to merge to parent transaction when removing since
calling getPendingTransaction will already return the DisplayContent's
transaction.

Also added a forced Transaction.apply() when removing the DisplayContent
since there's a chance it may no be reachable when
performSurfacePlacement is called to apply all transactions.

Test: EncodeVirtualDisplayWithCompositionTest#testVirtualDisplayRecycles
No longer leaks SurfaceControls
Fixes: 149526716

Change-Id: I14aa5b2e0e14d75fb0285a1b7a511235064f621c