String on Toast notification does not aligned to center

If length of the string being showed on Toast is short (less than 8 chars),
it is aligned to left rather than to center.
Added attribute android:layout_gravity="center_horizontal" on the TextView where Toast's message are shown to fix issue.

Change-Id: I77669fd8a11d1ec8f6b082348eb818671ce158bb
Signed-off-by: Taeho Kim <jyte82@gmail.com>