libgui: use read/writeStrongBinder

Still storing binder locally as a weak binder, but the plan is to remove
read/writeWeakBinder APIs. This is because weak binders can't be
promoted unless there is already a strong binder in a given process.

Bug: 137785053
Test: atest libgui_test (fails at same place as ToT: b/137791481)
Test: atest SurfaceFlinger_test (fails at same places as ToT: b/137875576)
Change-Id: I2d01aab95c4bd2e6db5e50055910cfa3064d9e52