dashboard: add android/386, android/amd64 builders
dashboard: add android/386, android/amd64 builders

Add an android/386 and an android/amd64 builder, both running on an
emulator. Use the new GOANDROID_ADB_FLAGS to direct the build to
either a device (android/arm, android/arm64) or an emulator
(android/386, android/amd64).

Change-Id: I8e25aa5efcbbb186c71c4e108a37ae11b0246c98
Reviewed-on: https://go-review.googlesource.com/88775
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build