Previous attempt to fix git sha on non-git build broke git build. (#3322)

This uses unusual code provided by Mikhail Lopatkin of Gradle Inc:
https://github.com/gradle/gradle/issues/23914#issuecomment-1431909019
It wraps the git sha function in a "value source". This allows it to interact correctly with configuration caching.
Because the code is longer than before, it is now broken out into its own file getGitSha.gradle.

app/build.gradle

@@ -5,19 +5,9 @@ plugins {
     alias(libs.plugins.kotlin.parcelize)
 }
 
-// For constructing gitSha only
-def getGitSha = {
-    try {  // Try-catch is necessary for build to work on non-git distributions
-        providers.exec {
-            commandLine 'git', 'rev-parse', 'HEAD'
-            executionResult.rethrowFailure() // Without this, sometimes it just stops immediately instead of throwing
-        }.standardOutput.asText.get().trim()
-    } catch (Exception e) {
-        "unknown"
-    }
-}
+apply from: 'getGitSha.gradle'
 
-final def gitSha = getGitSha()
+final def gitSha = ext.getGitSha()
 
 // The app name
 final def APP_NAME = "Tusky"