adb: fix flakiness in PTY shell protocol.

When a subprocess closes its PTY slave, the master fd will report
POLLHUP when polled. This leads to us prematurely tearing everything
down, without reading out output that's been written to the PTY.

Resolve this by waiting until the fd no longer reports POLLIN.

Bug: http://b/156551485
Bug: http://b/156552734
Test: `adb shell 'X=0; while /data/nativetest64/adbd_test/adbd_test --gtest_filter="ShellServiceTest.*Pty*" >/dev/null 2>&1; do X=$((X+1)); echo $X; done'` for 1000 iterations (failed within 20, previously)
Test: test_device.py
Change-Id: Ie591e0cafb532cd6cebdf6f356dc967565b5a2d9