InputWindow: 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_tests libinput_tests
Before and after:
libgui_test: Passed: 202, Failed: 1, Ignored: 0, Assumption Failed: 0
libinput_tests: Passed: 74, Failed: 0, Ignored: 0, Assumption Failed: 0

The ToT failure is:
[133/203] SurfaceTest#GetHdrSupport: FAILED (6ms)

Change-Id: I4e952672885a035174152286c529d82f55d48326