Rename getDataEnabled() to isDataEnabled().

The old API is reverted back to @SystemApi and marked deprecated to
avoid breaking any compilation.

Also clean up the Javadoc by making the permissions requirements
clearer and noting that this API does not take per-app data
restrictions into account.

Change-Id: I5baacc1fd7e75d54e1d6521c24f792f427d9e228
Test: TreeHugger build/boot
Fixes: 35766530