Remove BUILDTIME BuildConfig field

This commit removes the `BUILDTIME` field from `BuildConfig` across all modules. This change improves build caching and reproducibility by eliminating a value that changed with every build.

The `BUILDTIME` was previously used in `VERSION_DESCRIPTION_LONG` but is no longer included.

buildSrc/src/main/java/ProjectConfig.kt

@@ -1,6 +1,5 @@
 import java.io.File
 import java.io.FileInputStream
-import java.time.Instant
 import java.util.Properties
 
 object ProjectConfig {
@@ -35,8 +34,6 @@ fun lastCommitHash(): String = Runtime.getRuntime().exec("git rev-parse --short
     output.trim()
 }
 
-fun buildTime(): Instant = Instant.now()
-
 fun com.android.build.api.dsl.SigningConfig.setupCredentials(
     signingPropsPath: File? = null
 ) {