Remove a misleading "flush" function.

This patch removes LocalSocketImpl.flush().  In practice this function
was simply a wrapper around `Thread.sleep(10)`.  All direct calls to
this function have been removed.  The `flush()` function is still called
on several objects that wrap a SocketOutputStream.

This will make booting a device 20ms faster than it currently is.

Bug: 139192244
Test: Build -> flash -> boot -> launch app
Change-Id: I0a96f4bc72461670370f61e847349f32af5ac774