Minimize amount of time blocking inbound shortcut binder call during I/O

Since shortcuts for each individual packages are persisted in separate
files, we can postpone the I/O of these files to avoid holding the
synchronization lock for the entire duration of persisting these files.

Bug: 287558960
Test: atest CtsShortcutTestCases
Change-Id: I2e940d28a4dfcc74ff2b64a9e6b75b9e7766acf4