Revert "Merge pull request #21162 from yurenju/callscreen"

This reverts commit 061e9b84a9742fa9ad0519b7b98f1b50fc212be8, reversing
changes made to 34df8238f7367ac99b6897a073b9cc2f82f5371a.
Bug 1039943: make gallery exit if it goes to background while scanning on a 256mb device or smaller

Use < 512mb instead of <= 256mb as the threshold