Respect ANDROID_SERIAL environment variable used by ADB

When multiple Android devices are attached, the default behaviour of ADB
is to resolve a device number based on the presence of ANDROID_SERIAL if
the serial number is not explicitly passed by the -s parameter. This patch
emulates that behaviour in lldb's ADB platform connector

Differential Revision: http://reviews.llvm.org/D22052

llvm-svn: 274776