Do not generate aconfig_flags.textproto

The /<partition>/etc/aconfig_flags.textproto files are no longer needed
(all clients have migrated to use /<partition>/etc/aconfig_flags.pb).
Stop generating them.

Also update how an empty file is generated in case there are no flags
for a given partition. Previously

  echo "" > file

was used, which resulted in a file containing a newline, which caused
`printflags` to error out. Change the command to

  echo -n > file

to create an empty file.

Bug: 302452801
Bug: 304278614
Test: m nothing
Change-Id: I2f7d23df4e888046651b4155b75c5006d0b5e1d7