Detect if necessary to build inferior with -pie for Android.

Summary:
- Add target_is_android check (with cached results).
- Make android_device_api also cache results.
- Also removes the need to pass --env OS=Android when testing against Android.

Reviewers: sivachandra, tberghammer, clayborg, danalbert

Subscribers: chaoren, tberghammer, danalbert, srhines, lldb-commits

Differential Revision: http://reviews.llvm.org/D11309

llvm-svn: 242580