Export platform-tools/adb for incompatible_no_implicit_file_export

[hofbi]

Fixes #440

Adding exports_files to make

rules_android/rules/android_sdk_repository/template.bzl

Line 85 in 2985c25

actual = "platform-tools/adb",

visible. Alternatively, we could use the alias in

rules_android/toolchains/android/toolchain.bzl

Line 55 in 2985c25

default = "@androidsdk//:platform-tools/adb",

instead of the file.

[hofbi]

@ted-xie the CI don't seem to be related