Add offline docs target for support lib gradle build

Fixes standalone docs issues like references to hidden framework class
R.styleable. Removes reference to BackupAgent, which makes doclava choke
for some unknown reason.

Standalone online docs may now be built with ./gradlew :generateDocs

Bug: 28124434
Change-Id: If76ac9018be1d17f1258802d4ef3ce1ad67b4903

• Android.mk[diff]
• build.gradle[diff]
• compat/java/android/support/v4/app/ActivityOptionsCompat.java[diff]
• compat/java/android/support/v4/content/ContextCompat.java[diff]
• compat/java/android/support/v4/widget/TextViewCompat.java[diff]
• fragment/java/android/support/v4/app/Fragment.java[diff]
• frameworks/support/samples[Added - diff]
• transition/src/android/support/transition/Transition.java[diff]
• transition/src/android/support/transition/Visibility.java[diff]
• v7/appcompat/src/android/support/v7/app/ActionBar.java[diff]