Add back @UnsupportedAppUsage to IS_DEBUGGABLE

The annotation was removed when adding the constant to the public API,
but it was removed from the public API in favor of Build.isDebuggable() in
a later change.

Add back the @UnsupportedAppUsage annotation that should have been
added back when removing the constant from the public API.

Change-Id: Ic2fda01a8c0e90712a1b7b435443d32f7e226d33
Fixes: 187432390
Test: m