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.

Change-Id: I10424ad730c73516d16b0eb7318e8c9beea9b36b