Fix apk attachments so they can be installed

This introduces an in-memory cache of file paths for an attachment so
that we can install from a filepath instead of a content URI.
The cache also allows us to prevent duplicated files made if the user
hits save multiple times (the save button is disabled).

Bug: 3338997
Change-Id: I56651a55eef2f1a3f24b6d936c37736c4fc3d295
9 files changed