jni: Move customisation to properties

* libfmcust is a proprietary library provided by MediaTek
  which stores the custom config parameters used by the main
  FM library. Later on it was removed all together and instead
  the configs were hardcoded into MediaTek's libfmjni fork.

* The config can be easily retrieved from the exported CUST_get_cfg
  function in libfmcust. (or, the mangled version in libfmjni)

* With the following in mind, move the config values to properties
  where they can be easily configured.

* Since we don't hardcode anything this way, this also
  makes the library itself device independent.

Change-Id: Iecdb12f5d90099187934c57f58a3cfeb97d675bd
Signed-off-by: bengris32 <bengris32@protonmail.ch>