Dismiss PackageInstallerActivity when SimpleErrorDialog cancelled

Otherwise the translucent Activity sticks around until another back
press.

Bug: 187221431

Test: manual, adb shell am start \
    -a android.intent.action.INSTALL_PACKAGE \
    -d "file:///the-apk-that-wasnt.apk" \
    -n com.google.android.packageinstaller/com.android.packageinstaller.PackageInstallerActivity

Change-Id: I80ecf62247c62d76a832676ed52c68f9aefcb54e