Set GRADLE_USER_HOME when OUT_DIR is specified.
Set GRADLE_USER_HOME when OUT_DIR is specified.

On build server we don't want to use the default ~/.gradle/ home directory
as that get shared between beild and thus causing flakiness.

Test: OUT_DIR="../../out" ./gradlew assembleDebug
Change-Id: I02ea6228499285841848130c1405b8d4c0e049cd

AndroidX/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,aosp-mirror/platform_frameworks_support,AndroidX/androidx,aosp-mirror/platform_frameworks_support,androidx/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,aosp-mirror/platform_frameworks_support,androidx/androidx,AndroidX/androidx,androidx/androidx,aosp-mirror/platform_frameworks_support,aosp-mirror/platform_frameworks_support