Remove manual GC after unfreezing screen

The modern VM should be efficient enough to do GC automatically
on a dedicated thread. The removal can avoid significant random
delay by the explicitly GC on display thread.

About the legacy comment of "remote references", there is no strong
association with display change. There could have other cases to
produce many releasable remote references, but so far they are no
problem without an explicitly GC. Hence remove the manual GC to be
more general.

Bug: 255687140
Test: CtsWindowManagerDeviceTestCases
Change-Id: Ib620894b103ae33f1859a6e1d9627703eeb7b0ae