Remove flicker when multiple apps are added/removed/updated on widget tray

Bug: 36718342

1.The flicker was also happening partially because notifyWidgetProviderChanged
callback also made the entire widget list to update in addition to
packageManager update.

2. Now that adapter calls notifyItemInserted, Removed, the recycler
view uses it's internal animation to elegantly move items or insert
them. (added benefit!)

3. Added tests for WidgetsListAdapterTest

$ adb shell am instrument -w -e class com.android.launcher3.widget.WidgetsListAdapterTest com.google.android.apps.nexuslauncher.tests/android.support.test.runner.AndroidJUnitRunner

com.android.launcher3.widget.WidgetsListAdapterTest:.

Time: 0.337

OK (6 test)

Change-Id: I0818d546532631bf889fae560118decff64ec5a4
Signed-off-by: Hyunyoung Song <hyunyoungs@google.com>