[DO NOT MERGE] btif: Lighter weight file existence check

We switch from the heavier weight fopen() to using access() to
check for file existence and readability.  This avoids some of
the file opening overhead we don't need here.

Bug: b/117993149
Test: TreeHugger
Change-Id: Ib47b24e9d96803b7822535f9f566aa7818b38953
Merged-In: Ib47b24e9d96803b7822535f9f566aa7818b38953