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
