Enable selection action mode for Android M+.

Since I62963d225e6ea5d2ec9d8ebc8a6d73099f5d6c7f, we have disabled
selection action mode in the folder-name editing mode on Launcher,
mainly because it messed up the view on phones.  However that is no
longer the case in Android M and later, where floating action mode
is available.

With this CL, we start relying on the default floating action mode
in Android M and later.

Bug: 22210697
Bug: 28526978
Change-Id: Ie19e7844194acd86cdb013254a1923f0b1d78e11
(cherry picked from commit 1bd9a430)