Only include exported flags in android-flags.jar

The android-flags.jar file is consumed by apps built outside the
platform. These apps should only have access to exported flags (as
defined by the flag_declaration `is_exported` field).

Teach soong to filter out non-exported flags when generating
android-flags.jar.

Bug: 311151343
Test: m sdk dist # manually verify that $(gettop)/out/dist/android-flags.jar only contains exported flags
Change-Id: If66321fcffd920040dae3d99bd7ef69ff6925b18