Order entries in apex_info.pb

Adding the entries in apex_info.pb in sorted order makes it easier to
compare ota packages generated between the same source and target on
different computers/environments, ideally the generated ota zips should
be identical.

os.listdir() was used to find the apex files which should be
included in apex_info.pb. listdir() does not guarantee any order,
solution is to sort the result to ensure consistent order.

Bug: 278095305
Test: Manual. Confirm that apex_info.pb is generated sorted
    protoc --decode_raw < apex_info.pb
Change-Id: I2851d207f828a6ac31e63e20c9025965ac4b0eba