Ignore **/._* MacOS metadata files

MacOS creates these hidden files to hold extra file metadata specific to
their filesystem and needs when the filesystem format choosen doesn't
already do it for them.

These files don't need to be committed to version control. They're
tedious to remove, as the OS makes them each time you save a file.

Bug: 367386917
Flag: EXEMPT, only an update to our gitignore
Test: m com.android.btservices
Change-Id: Icee50b8f0d294136cab31ad8f5c6978a7fc3c6d9