apex: invoke `conv_linker_config validate` as validation

`conv_linker_config validate` command is used to validate the linker
configuration embedded in APEX to detect common mistakes.

For example, when used in APEX, linker configuration can't set
provideLibs/requireLibs. For APEX, there are
provideSharedLibs/requireSharedLibs in APEX manifest for that purpose.

One might make mistake by setting provideLibs in linker config.
Now, when these unsupported properties are set, there'll be build-time
error like:

 // set provideLibs key in com.android.art's linker config.
 $ m com.android.art
   ...image.apex/etc/linker.config.pb: provideLibs is set. Use provideSharedLibs in apex_manifest

Bug: 264341796
Test: m com.android.art (see above)
Change-Id: Ibaf7322616ad333569e6d721680f3d72243402a2