[Chromecast] Add <uses-sdk/> tag to Cast Shell manifest.

Without it, the manifest merger assumes we're using a library that
targets SDK version 1 and adds some permissions we don't need.

Bug: None
Test: check manifest from CastShellInternal.apk
Change-Id: I98b4597a21e94e5c2d920943df4fd26bf97a556c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1909641Reviewed-by: Luke Halliwell (slow) <halliwell@chromium.org>
Reviewed-by: Simeon Anfinrud <sanfin@chromium.org>
Commit-Queue: Thoren Paulson <thoren@chromium.org>
Auto-Submit: Thoren Paulson <thoren@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714558}

chromecast/BUILD.gn

@@ -659,6 +659,9 @@ if (is_android) {
     android_manifest = "$root_gen_dir/cast_shell_manifest/AndroidManifest.xml"
     android_manifest_dep = "//chromecast/browser/android:cast_shell_manifest"
 
+    min_sdk_version = 21
+    target_sdk_version = 28
+
     shared_libraries = [ "//chromecast/android:libcast_shell_android" ]
 
     product_config_java_packages = [ "org.chromium.chromecast.shell" ]

chromecast/browser/android/apk/AndroidManifest.xml.jinja2

@@ -8,6 +8,8 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
           package="org.chromium.chromecast.shell">
 
+    <uses-sdk android:minSdkVersion="21" />
+
     <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
     <uses-permission android:name="android.permission.INTERNET"/>
     <uses-permission android:name="android.permission.WAKE_LOCK"/>