Replace ANDROID_SDK_HOME with ANDROID_PREFS_ROOT for metalava

Fixes warnings:
Warning: Using ANDROID_SDK_HOME for the location of the '.android' preferences location is deprecated, please use ANDROID_PREFS_ROOT instead.
Support for ANDROID_SDK_HOME is deprecated and will be removed in 6.0

Test: m android_stubs_current
Change-Id: Ie8721dcda0578c670dfc796675ba43cda16883f6