envsetup.sh: Use prebuilt emulator binaries if available.

This patch changes the setpaths() function in envsetup.sh to probe
for prebuilts/android-emulator/<host>/, and prepend it to PATH if
it exists.

See https://googleplex-android-review.googlesource.com/#/c/459741/
for a related patch that adds the binaries.

BUG=13747402

Change-Id: I30794ea52f6dfc58908e6271f2c0da8e2f0b68e8