aconfig: Add script to generate finalized flag proto.

Takes /prebuilts/sdk/<level>/finalized-flags.txt and creates
aconfig_internal FinalizedFlags proto. For use in automatically adding
the sdk check in exported flag codegen.

Test
1. m convert_finalized_flags_to_proto
2. /path/to/aosp/out/host/linux-x86/bin/convert_finalized_flags_to_proto /path/to/aosp/prebuits/sdk > tmp_flags.pb
3. inspect proto for correctness

Next steps:
-Add genrule which uses this binary
-Read proto output in codegen

Test: manual, see above
Change-Id: I730a1c20347fe1a11fe7213fa25fffbe4ea3fc23