[Bugfix] Don't crash when MIME type is unknown.

When user tries to open a file of unknown MIME type through CertInstaller,
it just goes back and file manager is shown again. So we give user a
toast to let he or she knows what happened just now.

Bug: 218161567
Test: Manual
Change-Id: I2ff479343ea3c045eb6e3da7890b16c37b6c0212
1 file changed