Add android::base::GetExecutablePath, switch adb and fastboot over.

We'd long had two copies of this stuff, so rather than rewrite both
Linux versions to use android::base::Readlink, let's kill the duplication
too...

Bug: http://b/30988271
Change-Id: I4de58a94a22a4b1faf969a6fc70ca1560a4d5121