Update UpdateActivity.java

Settings.Secure.INSTALL_NON_MARKET_APPS has changed from deprecated to undeprecated (deprecating Settings.Global.INSTALL_NON_MARKET_APPS): https://developer.android.com/sdk/api_diff/21/changes/android.provider.Settings.Secure.html

Without this change, Android 5.0 cannot install updates from UpdateActivity
Merge pull request #63 from eromero83/patch-1

Use correct constant on API level 21