ANDROID: sdcardfs: Switch strcasecmp for internal call

This moves our uses of strcasecmp over to an internal call so we can
easily change implementations later if we so desire. Additionally,
we leverage qstr's where appropriate to save time on comparisons.

Signed-off-by: Daniel Rosenberg <drosen@google.com>
Change-Id: I32fdc4fd0cd3b7b735dcfd82f60a2516fd8272a5