fs_mgr_overlayfs: Make all string constants constexpr

According to https://abseil.io/tips/140, string constants should be
constexpr char array or string_view object. This avoids subtle bugs due
to the toolchain shuffling object initialization order between/within
compilation units.

string_view has bad interoperability between C APIs as many of those
functions require string values to be null-terminated. Thus we can only
rely on good old c-string constants.

This change groups all string constants together and change them all to
constexpr char array for consistent style.
Also remove some duplicated method definition.

Test: adb-remount-test
Change-Id: I1ed57e6dc24ce3750e72c5538c388a6872cd2b40