WM: Call Transaction#sanitize

Various elements of the Transaction interface require
a permission in order to apply. In particular the setTrustedOverlay
and setInputWindowInfo fields. These permission checks are
implemented by checking the PID and the UID of the process which
sent the transaction. Unfortunately widespread use of transaction
merging makes this inadequate. At the moment
IWindowSession#finishDrawing seems to be the only boundary on which
transactions move from client to system processes, and so we expose
a sanitize method and use it from there to resolve the situation
in an easily backportable way.

Moving forward it likely make sense to move security sensitive
interfaces off of Transaction. Most of the things behind permissions
currently are not truly security sensitive, more of just a request
not to use them.

It was also considered to sanitize transactions at all process
boundaries through writeToParcel, however this could be disruptive
as previously permissioned processes (WM and SysUI) could freely
exchange transactions. As the change needs to be backportable the
lowest risk option was chosen.

Bug: 213644870
Test: Existing tests pass
Change-Id: I8d4a0ebe0cdfaed7ff1ad862724d49a14ed04478