Adding wait for a device file before mount

Adding the function, fs_mgr_wait_for_file(), to ensure the
device file exists before to mount

Bug: 158064601
Merged-In: I74a68224073932773be18a79f9334d83ea5b6947
Change-Id: I74a68224073932773be18a79f9334d83ea5b6947
(cherry picked from commit 589b7dfd83a47eb0dbaa1f7bfa95b92c449f18c0)
1 file changed