Update AGP to 3.5.0

Also update the gitignore to ignore the '.cxx' directory AGP 3.5.0
uses for native build artifacts.

Signed-off-by: Harsh Shandilya <me@msfjarvis.dev>

.gitignore

@@ -6,7 +6,7 @@
 /captures/
 /local.properties
 .DS_Store
-.externalNativeBuild/
+.cxx/
 Thumbs.db
 build/
 *.apk

build.gradle

@@ -7,7 +7,7 @@ allprojects {
 
 buildscript {
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.3.1'
+        classpath 'com.android.tools.build:gradle:3.5.0'
     }
     repositories {
         google()