Fix '_' used as an identifier warning.

Use of '_' as an identifier might not be supported in releases after Java SE 8.
Fix ExtraText warning in note editor XML.
Fix ObsoleteLayoutParam warnings.
Fix GradleDependency by bumping compileSdkVersion to 21.
Fix Deprecated warning.

When a simple "android:editable=false" becomes android:clickable="false"
android:cursorVisible="false" android:focusable="false"
android:focusableInTouchMode="false".
Fix InefficientWeight warnings.