[Platform/Android] Read the adb server from an env variable if set

Summary:
The environment variable ANDROID_ADB_SERVER_PORT can be defined to have
adbd litsen on a different port. Teach lldb how to understand this via
simply checking the env var.

Reviewers: xiaobai, clayborg

Subscribers: srhines

Differential Revision: https://reviews.llvm.org/D66689

llvm-svn: 370106