Emulator: fix adbd qemu pipe partial write

It does happens that the adb_write only writes to the
qemu pipe partially which throws host side's adb backend
into confusion and crashes.

This CL replaces adb_write with WriteFdExactly;
adb_read with ReadFdExactly.

(cherry picked from commit f66c5938)

Change-Id: I684f5df79b1e3f00b4b7a2452c2712a73c15973c