Compiles wm shell against the platform APIs instead of SDK

WM Shell should be compiled against the platform APIs and thus should
leave sdk version empty. The value of platform_apis will be ignored when
module's type isn't android_app, remove it as well.

Bug: 161116823
Test: manual verified it could access to @hide APIs
Test: make WindowManager-Shell
Test: atest WindowManagerShellTest
Change-Id: I145b0d6352a6e10a2f4051a561e65c282374fa7c